pretty: add %(describe)

Add a format placeholder for describe output.  Implement it by actually
calling git describe, which is simple and guarantees correctness.  It's
intended to be used with $Format:...$ in files with the attribute
export-subst and git archive.  It can also be used with git log etc.,
even though that's going to be slow due to the fork for each commit.

Suggested-by: Eli Schwartz <[email protected]>
Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

Documentation/pretty-formats.txt

@@ -208,6 +208,8 @@ The placeholders are:
 '%cs':: committer date, short format (`YYYY-MM-DD`)
 '%d':: ref names, like the --decorate option of linkgit:git-log[1]
 '%D':: ref names without the " (", ")" wrapping.
+'%(describe)':: human-readable name, like linkgit:git-describe[1];
+		empty string for undescribable commits
 '%S':: ref name given on the command line by which the commit was reached
        (like `git log --source`), only works with `git log`
 '%e':: encoding

pretty.c

@@ -12,6 +12,7 @@
 #include "reflog-walk.h"
 #include "gpg-interface.h"
 #include "trailer.h"
+#include "run-command.h"
 
 static char *user_format;
 static struct cmt_fmt_map {
@@ -1214,6 +1215,22 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		return parse_padding_placeholder(placeholder, c);
 	}
 
+	if (skip_prefix(placeholder, "(describe)", &arg)) {
+		struct child_process cmd = CHILD_PROCESS_INIT;
+		struct strbuf out = STRBUF_INIT;
+		struct strbuf err = STRBUF_INIT;
+
+		cmd.git_cmd = 1;
+		strvec_push(&cmd.args, "describe");
+		strvec_push(&cmd.args, oid_to_hex(&commit->object.oid));
+		pipe_command(&cmd, NULL, 0, &out, 0, &err, 0);
+		strbuf_rtrim(&out);
+		strbuf_addbuf(sb, &out);
+		strbuf_release(&out);
+		strbuf_release(&err);
+		return arg - placeholder;
+	}
+
 	/* these depend on the commit */
 	if (!commit->object.parsed)
 		parse_object(the_repository, &commit->object.oid);

t/t4205-log-pretty-formats.sh

@@ -962,4 +962,14 @@ test_expect_success 'log --pretty=reference is colored appropriately' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(describe) vs git describe' '
+	git log --format="%H" | while read hash
+	do
+		echo "$hash $(git describe $hash)"
+	done >expect &&
+	git log --format="%H %(describe)" >actual 2>err &&
+	test_cmp expect actual &&
+	test_must_be_empty err
+'
+
 test_done