freshen_file(): use NULL `times' for implicit current-time

Update freshen_file() to use a NULL `times', semantically equivalent to
the currently setup, with an explicit `actime' and `modtime' set to the
"current time", but with the advantage that it works with other files
not owned by the current user.

Fixes an issue on shared repos with a split index, where eventually a
user's operation creates a shared index, and another user will later do
an operation that will try to update its freshness, but will instead
raise a warning:
  $ git status
  warning: could not freshen shared index '.git/sharedindex.bd736fa10e0519593fefdb2aec253534470865b2'

Signed-off-by: Luciano Miguel Ferreira Rocha <luciano.rocha@booking.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1-file.c

@@ -868,9 +868,7 @@ void prepare_alt_odb(struct repository *r)
 /* Returns 1 if we have successfully freshened the file, 0 otherwise. */
 static int freshen_file(const char *fn)
 {
-	struct utimbuf t;
-	t.actime = t.modtime = time(NULL);
-	return !utime(fn, &t);
+	return !utime(fn, NULL);
 }
 
 /*