rebase -i: only write fixup-message when it's needed

The file "$GIT_DIR/rebase-merge/fixup-message" is only used for fixup
commands, there's no point in writing it for squash commands as it is
immediately deleted.

Signed-off-by: Phillip Wood <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Charvi Mendiratta <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phillipwood

sequencer.c

@@ -1757,11 +1757,10 @@ static int update_squash_messages(struct repository *r,
 			return error(_("could not read HEAD's commit message"));
 
 		find_commit_subject(head_message, &body);
-		if (write_message(body, strlen(body),
-				  rebase_path_fixup_msg(), 0)) {
+		if (command == TODO_FIXUP && write_message(body, strlen(body),
+							rebase_path_fixup_msg(), 0) < 0) {
 			unuse_commit_buffer(head_commit, head_message);
-			return error(_("cannot write '%s'"),
-				     rebase_path_fixup_msg());
+			return error(_("cannot write '%s'"), rebase_path_fixup_msg());
 		}
 
 		strbuf_addf(&buf, "%c ", comment_line_char);