builtin/commit.c: memoize git-path for COMMIT_EDITMSG

This is a follow up commit for f932729 (memoize common git-path
"constant" files, 10-Aug-2015).

The many function calls to git_path() are replaced by
git_path_commit_editmsg() and which thus eliminates the need to repeatedly
compute the location of "COMMIT_EDITMSG".

Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -92,8 +92,9 @@ N_("If you wish to skip this commit, use:\n"
 "Then \"git cherry-pick --continue\" will resume cherry-picking\n"
 "the remaining commits.\n");
 
+static GIT_PATH_FUNC(git_path_commit_editmsg, "COMMIT_EDITMSG")
+
 static const char *use_message_buffer;
-static const char commit_editmsg[] = "COMMIT_EDITMSG";
 static struct lock_file index_lock; /* real index */
 static struct lock_file false_lock; /* used only for partial commits */
 static enum {
@@ -772,9 +773,9 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		hook_arg2 = "";
 	}
 
-	s->fp = fopen_for_writing(git_path(commit_editmsg));
+	s->fp = fopen_for_writing(git_path_commit_editmsg());
 	if (s->fp == NULL)
-		die_errno(_("could not open '%s'"), git_path(commit_editmsg));
+		die_errno(_("could not open '%s'"), git_path_commit_editmsg());
 
 	/* Ignore status.displayCommentPrefix: we do need comments in COMMIT_EDITMSG. */
 	old_display_comment_prefix = s->display_comment_prefix;
@@ -951,23 +952,23 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	}
 
 	if (run_commit_hook(use_editor, index_file, "prepare-commit-msg",
-			    git_path(commit_editmsg), hook_arg1, hook_arg2, NULL))
+			    git_path_commit_editmsg(), hook_arg1, hook_arg2, NULL))
 		return 0;
 
 	if (use_editor) {
 		char index[PATH_MAX];
 		const char *env[2] = { NULL };
 		env[0] =  index;
 		snprintf(index, sizeof(index), "GIT_INDEX_FILE=%s", index_file);
-		if (launch_editor(git_path(commit_editmsg), NULL, env)) {
+		if (launch_editor(git_path_commit_editmsg(), NULL, env)) {
 			fprintf(stderr,
 			_("Please supply the message using either -m or -F option.\n"));
 			exit(1);
 		}
 	}
 
 	if (!no_verify &&
-	    run_commit_hook(use_editor, index_file, "commit-msg", git_path(commit_editmsg), NULL)) {
+	    run_commit_hook(use_editor, index_file, "commit-msg", git_path_commit_editmsg(), NULL)) {
 		return 0;
 	}
 
@@ -1738,7 +1739,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	/* Finally, get the commit message */
 	strbuf_reset(&sb);
-	if (strbuf_read_file(&sb, git_path(commit_editmsg), 0) < 0) {
+	if (strbuf_read_file(&sb, git_path_commit_editmsg(), 0) < 0) {
 		int saved_errno = errno;
 		rollback_index_files();
 		die(_("could not read commit message: %s"), strerror(saved_errno));