commit/status: check $GIT_DIR/MERGE_HEAD only once

The code checked for the MERGE_HEAD file to see if we were about
to commit a merge twice in the codepath; also one of them used a
variable merge_head_sha1[] which was set but was never used.

Just check it once, but do so also in "git status", too, as
we will be using this for status generation in the next patch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -36,7 +36,7 @@ static const char * const builtin_status_usage[] = {
 	NULL
 };
 
-static unsigned char head_sha1[20], merge_head_sha1[20];
+static unsigned char head_sha1[20];
 static char *use_message_buffer;
 static const char commit_editmsg[] = "COMMIT_EDITMSG";
 static struct lock_file index_lock; /* real index */
@@ -319,7 +319,7 @@ static char *prepare_index(int argc, const char **argv, const char *prefix, int
 	 */
 	commit_style = COMMIT_PARTIAL;
 
-	if (file_exists(git_path("MERGE_HEAD")))
+	if (in_merge)
 		die("cannot do a partial commit during a merge.");
 
 	memset(&partial, 0, sizeof(partial));
@@ -758,9 +758,6 @@ static int parse_and_validate_options(int argc, const char *argv[],
 	if (get_sha1("HEAD", head_sha1))
 		initial_commit = 1;
 
-	if (!get_sha1("MERGE_HEAD", merge_head_sha1))
-		in_merge = 1;
-
 	/* Sanity check options */
 	if (amend && initial_commit)
 		die("You have nothing to amend.");
@@ -951,6 +948,7 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 
 	wt_status_prepare(&s);
 	git_config(git_status_config, &s);
+	in_merge = file_exists(git_path("MERGE_HEAD"));
 	argc = parse_options(argc, argv, prefix,
 			     builtin_status_options,
 			     builtin_status_usage, 0);
@@ -1057,10 +1055,10 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	wt_status_prepare(&s);
 	git_config(git_commit_config, &s);
+	in_merge = file_exists(git_path("MERGE_HEAD"));
 
 	if (s.use_color == -1)
 		s.use_color = git_use_color_default;
-
 	argc = parse_and_validate_options(argc, argv, builtin_commit_usage,
 					  prefix, &s);
 	if (dry_run) {