Merge branch 'bp/post-index-change-hook'

A new hook "post-index-change" is called when the on-disk index
file changes, which can help e.g. a virtualized working tree
implementation.

* bp/post-index-change-hook:
  read-cache: add post-index-change hook

builtin/update-index.c

@@ -1082,6 +1082,8 @@ 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.