cache: add fake_lstat()

At times, we may already know that a path represented by a
cache_entry ce has no changes via some out-of-line means, like
fsmonitor, and yet need the control to go through a codepath that
requires us to have "struct stat" obtained by lstat() on the path,
for various purposes (e.g. "ie_match_stat()" wants cached stat-info
is still current wrt "struct stat", "diff" wants to know st_mode).

The callers of lstat() on a tracked file, when its cache_entry knows
it is up-to-date, can instead call this helper to pretend that it
called lstat() by faking the "struct stat" information.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

statinfo.h

@@ -46,6 +46,14 @@ struct stat_validity {
  */
 void fill_stat_data(struct stat_data *sd, struct stat *st);
 
+/*
+ * The inverse of the above.  When we know the cache_entry that
+ * contains sd is up-to-date, but still need to pretend we called
+ * lstat() to learn that fact, this function fills "st" enough to
+ * fool ie_match_stat().
+ */
+void fake_lstat_data(const struct stat_data *sd, struct stat *st);
+
 /*
  * Return 0 if st is consistent with a file not having been changed
  * since sd was filled.  If there are differences, return a