built-in add -i: refresh the index before running `status`

This is what the Perl version does, and therefore it is what the
built-in version should do, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

add-interactive.c

@@ -258,7 +258,9 @@ int run_add_i(struct repository *r, const struct pathspec *ps)
 		    _("staged"), _("unstaged"), _("path"));
 	opts.header = header.buf;
 
-	res = run_status(r, ps, &files, &opts);
+	repo_refresh_and_write_index(r, REFRESH_QUIET, 1);
+	if (run_status(r, ps, &files, &opts) < 0)
+		res = -1;
 
 	release_file_list(&files);
 	strbuf_release(&print_file_item_data.buf);

repository.c

@@ -275,3 +275,22 @@ int repo_hold_locked_index(struct repository *repo,
 		BUG("the repo hasn't been setup");
 	return hold_lock_file_for_update(lf, repo->index_file, flags);
 }
+
+int repo_refresh_and_write_index(struct repository *r,
+				 unsigned int flags, int gentle)
+{
+	struct lock_file lock_file = LOCK_INIT;
+	int fd;
+
+	if (repo_read_index_preload(r, NULL, 0) < 0)
+		return error(_("could not read index"));
+	fd = repo_hold_locked_index(r, &lock_file, 0);
+	if (!gentle && fd < 0)
+		return error(_("could not lock index for writing"));
+	refresh_index(r->index, flags, NULL, NULL, NULL);
+	if (0 <= fd)
+		repo_update_index_if_able(r, &lock_file);
+	rollback_lock_file(&lock_file);
+
+	return 0;
+}

repository.h

@@ -157,5 +157,12 @@ int repo_read_index_unmerged(struct repository *);
  */
 void repo_update_index_if_able(struct repository *, struct lock_file *);
 
+/*
+ * Refresh the index and write it out. If the index file could not be
+ * locked, error out, except in gentle mode. The flags will be passed
+ * through to refresh_index().
+ */
+int repo_refresh_and_write_index(struct repository *r,
+				 unsigned int flags, int gentle);
 
 #endif /* REPOSITORY_H */