[PATCH 2/2] commit: add support for --fixup <commit> -m"<extra message>"

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Add support for supplying the -m option with --fixup. Doing so has
errored out ever since --fixup was introduced. Before this, the only
way to amend the fixup message while committing was to use --edit and
amend it in the editor.

The use-case for this feature is one of:

 * Leaving a quick note to self when creating a --fixup commit when
   it's not self-evident why the commit should be squashed without a
   note into another one.

 * (Ab)using the --fixup feature to "fix up" commits that have already
   been pushed to a branch that doesn't allow non-fast-forwards,
   i.e. just noting "this should have been part of that other commit",
   and if the history ever got rewritten in the future the two should
   be combined.

   In such a case you might want to leave a small message,
   e.g. "forgot this part, which broke XYZ".

When the --fixup option was initially added the "Option -m cannot be
combined" error was expanded from -c, -C and -F to also include
--fixup[1]

Those options could also support combining with -m, but given what
they do I can't think of a good use-case for doing that, so I have not
made the more invasive change of splitting up the logic in commit.c to
first act on those, and then on -m options.

1. d71b8ba7c9 ("commit: --fixup option for use with rebase
   --autosquash", 2010-11-02)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/git-commit.txt | 4 ++--
 builtin/commit.c             | 8 +++++---
 t/t7500-commit.sh            | 9 ++++++++-
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/Documentation/git-commit.txt b/Documentation/git-commit.txt
index df83176314..4489677fd1 100644
--- a/Documentation/git-commit.txt
+++ b/Documentation/git-commit.txt
@@ -145,8 +145,8 @@ OPTIONS
 	If multiple `-m` options are given, their values are
 	concatenated as separate paragraphs.
 +
-Combining the `-m` option and any of `-c`, `-C`, `-F` or `--fixup`
-will result in an error.
+Combining the `-m` option and any of `-c`, `-C` or `-F` will result in
+an error.
 
 -t <file>::
 --template=<file>::
diff --git a/builtin/commit.c b/builtin/commit.c
index 8a87701414..4e68394391 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -701,7 +701,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		}
 	}
 
-	if (have_option_m) {
+	if (have_option_m && !fixup_message) {
 		strbuf_addbuf(&sb, &message);
 		hook_arg1 = "message";
 	} else if (logfile && !strcmp(logfile, "-")) {
@@ -731,6 +731,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		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);
 		hook_arg1 = "message";
 	} else if (!stat(git_path_merge_msg(), &statbuf)) {
 		/*
@@ -1197,8 +1199,8 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		f++;
 	if (f > 1)
 		die(_("Only one of -c/-C/-F/--fixup can be used."));
-	if (have_option_m && f > 0)
-		die((_("Option -m cannot be combined with -c/-C/-F/--fixup.")));
+	if (have_option_m && (edit_message || use_message || logfile))
+		die((_("Option -m cannot be combined with -c/-C/-F.")));
 	if (f || have_option_m)
 		template_file = NULL;
 	if (edit_message)
diff --git a/t/t7500-commit.sh b/t/t7500-commit.sh
index 5739d3ed23..2d95778b74 100755
--- a/t/t7500-commit.sh
+++ b/t/t7500-commit.sh
@@ -272,6 +272,14 @@ test_expect_success 'commit --fixup provides correct one-line commit message' '
 	commit_msg_is "fixup! target message subject line"
 '
 
+test_expect_success 'commit --fixup -m"something" -m"extra"' '
+	commit_for_rebase_autosquash_setup &&
+	git commit --fixup HEAD~1 -m"something" -m"extra" &&
+	commit_msg_is "fixup! target message subject linesomething
+
+extra"
+'
+
 test_expect_success 'commit --squash works with -F' '
 	commit_for_rebase_autosquash_setup &&
 	echo "log message from file" >msgfile &&
@@ -325,7 +333,6 @@ test_expect_success 'invalid message options when using --fixup' '
 	test_must_fail git commit --fixup HEAD~1 --squash HEAD~2 &&
 	test_must_fail git commit --fixup HEAD~1 -C HEAD~2 &&
 	test_must_fail git commit --fixup HEAD~1 -c HEAD~2 &&
-	test_must_fail git commit --fixup HEAD~1 -m "cmdline message" &&
 	test_must_fail git commit --fixup HEAD~1 -F log
 '
 
-- 
2.15.1.424.g9478a66081