[PATCH v2 3/4] merge: make prepare_to_commit responsible for write_merge_state

[Ryan Biesemeyer <ryan@yaauie.com>]

When merging, make the prepare-commit-msg hook have access to the merge
state in order to make decisions about the commit message it is preparing.

Since `abort_commit` is *only* called after `write_merge_state`, and a
successful commit *always* calls `drop_save` to clear the saved state, this
change effectively ensures that the merge state is also available to the
prepare-commit-msg hook and while the message is being edited.

Signed-off-by: Ryan Biesemeyer <ryan@yaauie.com>
---
 builtin/merge.c                    |  3 ++-
 t/t7505-prepare-commit-msg-hook.sh | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index 4941a6c..b7bfc9c 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -802,7 +802,6 @@ static void abort_commit(struct commit_list *remoteheads, const char *err_msg)
 		error("%s", err_msg);
 	fprintf(stderr,
 		_("Not committing merge; use 'git commit' to complete the merge.\n"));
-	write_merge_state(remoteheads);
 	exit(1);
 }
 
@@ -816,6 +815,8 @@ N_("Please enter a commit message to explain why this merge is necessary,\n"
 static void prepare_to_commit(struct commit_list *remoteheads)
 {
 	struct strbuf msg = STRBUF_INIT;
+
+	write_merge_state(remoteheads);
 	strbuf_addbuf(&msg, &merge_msg);
 	strbuf_addch(&msg, '\n');
 	if (0 < option_edit)
diff --git a/t/t7505-prepare-commit-msg-hook.sh b/t/t7505-prepare-commit-msg-hook.sh
index 697ecc0..7ccf870 100755
--- a/t/t7505-prepare-commit-msg-hook.sh
+++ b/t/t7505-prepare-commit-msg-hook.sh
@@ -183,4 +183,25 @@ test_expect_success 'with failing hook (merge)' '
   )
 '
 
+test_expect_success 'should have MERGE_HEAD (merge)' '
+
+	git checkout -B other HEAD@{1} &&
+	echo "more" >> file &&
+	git add file &&
+	rm -f "$HOOK" &&
+	git commit -m other &&
+	git checkout - &&
+	write_script "$HOOK" <<-EOF &&
+	if [ -s "$(git rev-parse --git-dir)/MERGE_HEAD" ]; then
+		exit 0
+	else
+		exit 1
+	fi
+	EOF
+	git merge other &&
+	test "`git log -1 --pretty=format:%s`" = "Merge branch '"'"'other'"'"'" &&
+	test ! -s "$(git rev-parse --git-dir)/MERGE_HEAD"
+
+'
+
 test_done
-- 
1.8.5