move run_commit_hook() to libgit and use it there

This function was declared in commit.h but was implemented in
builtin/commit.c so was not part of libgit. Move it to libgit so we can
use it in the sequencer. This simplifies the implementation of
run_prepare_commit_msg_hook() and will be used in the next commit.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/sequencer.c b/sequencer.c
index 2adcf5a..cdc0d1d 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -1127,25 +1127,22 @@ static int run_prepare_commit_msg_hook(struct repository *r,
 				       struct strbuf *msg,
 				       const char *commit)
 {
-	struct argv_array hook_env = ARGV_ARRAY_INIT;
-	int ret;
-	const char *name;
+	int ret = 0;
+	const char *name, *arg1 = NULL, *arg2 = NULL;
 
 	name = git_path_commit_editmsg();
 	if (write_message(msg->buf, msg->len, name, 0))
 		return -1;
 
-	argv_array_pushf(&hook_env, "GIT_INDEX_FILE=%s", r->index_file);
-	argv_array_push(&hook_env, "GIT_EDITOR=:");
-	if (commit)
-		ret = run_hook_le(hook_env.argv, "prepare-commit-msg", name,
-				  "commit", commit, NULL);
-	else
-		ret = run_hook_le(hook_env.argv, "prepare-commit-msg", name,
-				  "message", NULL);
-	if (ret)
+	if (commit) {
+		arg1 = "commit";
+		arg2 = commit;
+	} else {
+		arg1 = "message";
+	}
+	if (run_commit_hook(0, r->index_file, "prepare-commit-msg", name,
+			    arg1, arg2, NULL))
 		ret = error(_("'prepare-commit-msg' hook failed"));
-	argv_array_clear(&hook_env);
 
 	return ret;
 }