sequencer: use the new hook API for the simpler "post-rewrite" call

Change the invocation of the "post-rewrite" hook added in
795160457d (sequencer (rebase -i): run the post-rewrite hook, if
needed, 2017-01-02) to use the new hook API.

This leaves the more complex "post-rewrite" invocation added in
a87a6f3c98 (commit: move post-rewrite code to libgit, 2017-11-17)
here in sequencer.c unconverted.

Here we can pass in a file's via the "in" file descriptor, in that
case we don't have a file, but will need to write_in_full() to an "in"
provide by the API. Support for that will be added to the hook API in
the future, but we're not there yet.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -4834,8 +4834,7 @@ cleanup_head_ref:
 		if (!stat(rebase_path_rewritten_list(), &st) &&
 				st.st_size > 0) {
 			struct child_process child = CHILD_PROCESS_INIT;
-			const char *post_rewrite_hook =
-				find_hook("post-rewrite");
+			struct run_hooks_opt hook_opt = RUN_HOOKS_OPT_INIT;
 
 			child.in = open(rebase_path_rewritten_list(), O_RDONLY);
 			child.git_cmd = 1;
@@ -4845,18 +4844,9 @@ cleanup_head_ref:
 			/* we don't care if this copying failed */
 			run_command(&child);
 
-			if (post_rewrite_hook) {
-				struct child_process hook = CHILD_PROCESS_INIT;
-
-				hook.in = open(rebase_path_rewritten_list(),
-					O_RDONLY);
-				hook.stdout_to_stderr = 1;
-				hook.trace2_hook_name = "post-rewrite";
-				strvec_push(&hook.args, post_rewrite_hook);
-				strvec_push(&hook.args, "rebase");
-				/* we don't care if this hook failed */
-				run_command(&hook);
-			}
+			hook_opt.path_to_stdin = rebase_path_rewritten_list();
+			strvec_push(&hook_opt.args, "rebase");
+			run_hooks_opt("post-rewrite", &hook_opt);
 		}
 		apply_autostash(rebase_path_autostash());