read-cache: add post-index-change hook

Add a post-index-change hook that is invoked after the index is written in
do_write_locked_index().

This hook is meant primarily for notification, and cannot affect
the outcome of git commands that trigger the index write.

The hook is passed a flag to indicate whether the working directory was
updated or not and a flag indicating if a skip-worktree bit could have
changed.  These flags enable the hook to optimize its response to the
index change notification.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -380,6 +380,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 			int flags = quiet ? REFRESH_QUIET : REFRESH_IN_PORCELAIN;
 			if (read_from_tree(&pathspec, &oid, intent_to_add))
 				return 1;
+			the_index.updated_skipworktree = 1;
 			if (!quiet && get_git_work_tree()) {
 				uint64_t t_begin, t_delta_in_ms;
 

builtin/update-index.c

@@ -1071,6 +1071,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	if (entries < 0)
 		die("cache corrupted");
 
+	the_index.updated_skipworktree = 1;
+
 	/*
 	 * Custom copy of parse_options() because we want to handle
 	 * filename arguments as they come.