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>

read-cache-ll.h

@@ -436,6 +436,14 @@ int match_stat_data_racy(const struct index_state *istate,
 
 void fill_stat_cache_info(struct index_state *istate, struct cache_entry *ce, struct stat *st);
 
+/*
+ * Fill members of st by members of sd enough to convince match_stat()
+ * to consider that they match.  It should be usable as a replacement
+ * for lstat() for a tracked path that is known to be up-to-date via
+ * some out-of-line means (like fsmonitor).
+ */
+int fake_lstat(const struct cache_entry *ce, struct stat *st);
+
 #define REFRESH_REALLY                   (1 << 0) /* ignore_valid */
 #define REFRESH_UNMERGED                 (1 << 1) /* allow unmerged */
 #define REFRESH_QUIET                    (1 << 2) /* be quiet about it */