commit: add amend suboption to --fixup to create amend! commit

`git commit --fixup=amend:<commit>` will create an "amend!" commit.
The resulting commit message subject will be "amend! ..." where
"..." is the subject line of <commit> and the initial message
body will be <commit>'s message.

The "amend!" commit when rebased with --autosquash will fixup the
contents and replace the commit message of <commit> with the
"amend!" commit's message body.

In order to prevent rebase from creating commits with an empty
message we refuse to create an "amend!" commit if commit message
body is empty.

Mentored-by: Christian Couder <[email protected]>
Mentored-by: Phillip Wood <[email protected]>
Helped-by: Junio C Hamano <[email protected]>
Helped-by: Eric Sunshine <[email protected]>
Signed-off-by: Charvi Mendiratta <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: charvi-077

builtin/commit.c

@@ -105,7 +105,8 @@ static const char *template_file;
  */
 static const char *author_message, *author_message_buffer;
 static char *edit_message, *use_message;
-static char *fixup_message, *squash_message;
+static char *fixup_message, *fixup_commit, *squash_message;
+static const char *fixup_prefix;
 static int all, also, interactive, patch_interactive, only, amend, signoff;
 static int edit_flag = -1; /* unspecified */
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
@@ -357,7 +358,8 @@ static const char *prepare_index(const char **argv, const char *prefix,
 		die(_("--pathspec-file-nul requires --pathspec-from-file"));
 	}
 
-	if (!pathspec.nr && (also || (only && !amend && !allow_empty)))
+	if (!pathspec.nr && (also || (only && !allow_empty &&
+	    (!amend || (fixup_message && strcmp(fixup_prefix, "amend"))))))
 		die(_("No paths with --include/--only does not make sense."));
 
 	if (read_cache_preload(&pathspec) < 0)
@@ -681,6 +683,22 @@ static void adjust_comment_line_char(const struct strbuf *sb)
 	comment_line_char = *p;
 }
 
+static void prepare_amend_commit(struct commit *commit, struct strbuf *sb,
+				struct pretty_print_context *ctx)
+{
+	const char *buffer, *subject, *fmt;
+
+	buffer = get_commit_buffer(commit, NULL);
+	find_commit_subject(buffer, &subject);
+	/*
+	 * If we amend the 'amend!' commit then we don't want to
+	 * duplicate the subject line.
+	 */
+	fmt = starts_with(subject, "amend!") ? "%b" : "%B";
+	format_commit_message(commit, fmt, sb, ctx);
+	unuse_commit_buffer(commit, buffer);
+}
+
 static int prepare_to_commit(const char *index_file, const char *prefix,
 			     struct commit *current_head,
 			     struct wt_status *s,
@@ -745,15 +763,33 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	} else if (fixup_message) {
 		struct pretty_print_context ctx = {0};
 		struct commit *commit;
-		commit = lookup_commit_reference_by_name(fixup_message);
+		char *fmt;
+		commit = lookup_commit_reference_by_name(fixup_commit);
 		if (!commit)
-			die(_("could not lookup commit %s"), fixup_message);
+			die(_("could not lookup commit %s"), fixup_commit);
 		ctx.output_encoding = get_commit_output_encoding();
-		format_commit_message(commit, "fixup! %s\n\n",
-				      &sb, &ctx);
-		if (have_option_m)
-			strbuf_addbuf(&sb, &message);
+		fmt = xstrfmt("%s! %%s\n\n", fixup_prefix);
+		format_commit_message(commit, fmt, &sb, &ctx);
+		free(fmt);
 		hook_arg1 = "message";
+
+		/*
+		 * Only `-m` commit message option is checked here, as
+		 * it supports `--fixup` to append the commit message.
+		 *
+		 * The other commit message options `-c`/`-C`/`-F` are
+		 * incompatible with all the forms of `--fixup` and
+		 * have already errored out while parsing the `git commit`
+		 * options.
+		 */
+		if (have_option_m && !strcmp(fixup_prefix, "fixup"))
+			strbuf_addbuf(&sb, &message);
+
+		if (!strcmp(fixup_prefix, "amend")) {
+			if (have_option_m)
+				die(_("cannot combine -m with --fixup:%s"), fixup_message);
+			prepare_amend_commit(commit, &sb, &ctx);
+		}
 	} else if (!stat(git_path_merge_msg(the_repository), &statbuf)) {
 		size_t merge_msg_start;
 
@@ -1170,7 +1206,7 @@ static int parse_and_validate_options(int argc, const char *argv[],
 	if (force_author && renew_authorship)
 		die(_("Using both --reset-author and --author does not make sense"));
 
-	if (logfile || have_option_m || use_message || fixup_message)
+	if (logfile || have_option_m || use_message)
 		use_editor = 0;
 	if (0 <= edit_flag)
 		use_editor = edit_flag;
@@ -1227,6 +1263,36 @@ static int parse_and_validate_options(int argc, const char *argv[],
 
 	if (also + only + all + interactive > 1)
 		die(_("Only one of --include/--only/--all/--interactive/--patch can be used."));
+
+	if (fixup_message) {
+		/*
+		 * We limit --fixup's suboptions to only alpha characters.
+		 * If the first character after a run of alpha is colon,
+		 * then the part before the colon may be a known suboption
+		 * name `amend` or a misspelt suboption name. In this case,
+		 * we treat it as --fixup=<suboption>:<arg>.
+		 *
+		 * Otherwise, we are dealing with --fixup=<commit>.
+		 */
+		char *p = fixup_message;
+		while (isalpha(*p))
+			p++;
+		if (p > fixup_message && *p == ':') {
+			*p = '\0';
+			fixup_commit = p + 1;
+			if (!strcmp("amend", fixup_message)) {
+				fixup_prefix = "amend";
+				allow_empty = 1;
+			} else {
+				die(_("unknown option: --fixup=%s:%s"), fixup_message, fixup_commit);
+			}
+		} else {
+			fixup_commit = fixup_message;
+			fixup_prefix = "fixup";
+			use_editor = 0;
+		}
+	}
+
 	cleanup_mode = get_cleanup_mode(cleanup_arg, use_editor);
 
 	handle_untracked_files_arg(s);
@@ -1504,7 +1570,11 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		OPT_CALLBACK('m', "message", &message, N_("message"), N_("commit message"), opt_parse_m),
 		OPT_STRING('c', "reedit-message", &edit_message, N_("commit"), N_("reuse and edit message from specified commit")),
 		OPT_STRING('C', "reuse-message", &use_message, N_("commit"), N_("reuse message from specified commit")),
-		OPT_STRING(0, "fixup", &fixup_message, N_("commit"), N_("use autosquash formatted message to fixup specified commit")),
+		/*
+		 * TRANSLATORS: Leave "[amend:]" as-is, and
+		 * only translate <commit>.
+		 */
+		OPT_STRING(0, "fixup", &fixup_message, N_("[amend:]commit"), N_("use autosquash formatted message to fixup or amend specified commit")),
 		OPT_STRING(0, "squash", &squash_message, N_("commit"), N_("use autosquash formatted message to squash specified commit")),
 		OPT_BOOL(0, "reset-author", &renew_authorship, N_("the commit is authored by me now (used with -C/-c/--amend)")),
 		OPT_BOOL('s', "signoff", &signoff, N_("add a Signed-off-by trailer")),
@@ -1663,6 +1733,19 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		exit(1);
 	}
 
+	if (fixup_message && starts_with(sb.buf, "amend! ") &&
+	    !allow_empty_message) {
+		struct strbuf body = STRBUF_INIT;
+		size_t len = commit_subject_length(sb.buf);
+		strbuf_addstr(&body, sb.buf + len);
+		if (message_is_empty(&body, cleanup_mode)) {
+			rollback_index_files();
+			fprintf(stderr, _("Aborting commit due to empty commit message body.\n"));
+			exit(1);
+		}
+		strbuf_release(&body);
+	}
+
 	if (amend) {
 		const char *exclude_gpgsig[3] = { "gpgsig", "gpgsig-sha256", NULL };
 		extra = read_commit_extra_headers(current_head, exclude_gpgsig);