fsmonitor: refactor untracked-cache invalidation

Update fsmonitor_refresh_callback() to use the new
untracked_cache_invalidate_trimmed_path() to invalidate
the cache using the observed pathname without needing to
modify the caller's buffer.

Previously, we modified the caller's buffer when the observed pathname
contained a trailing slash (and did not restore it).  This wasn't a
problem for the single use-case caller, but felt dirty nontheless.  In
a later commit we will want to invalidate case-corrected versions of
the pathname (using possibly borrowed pathnames from the name-hash or
dir-name-hash) and we may not want to keep the tradition of altering
the passed-in pathname.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsmonitor.c

@@ -271,21 +271,16 @@ static void fsmonitor_refresh_callback(struct index_state *istate, char *name)
 
 	if (name[len - 1] == '/') {
 		handle_path_with_trailing_slash(istate, name, pos);
-
-		/*
-		 * We need to remove the traling "/" from the path
-		 * for the untracked cache.
-		 */
-		name[len - 1] = '\0';
 	} else {
 		handle_path_without_trailing_slash(istate, name, pos);
 	}
 
 	/*
 	 * Mark the untracked cache dirty even if it wasn't found in the index
-	 * as it could be a new untracked file.
+	 * as it could be a new untracked file.  (Let the untracked cache
+	 * layer silently deal with any trailing slash.)
 	 */
-	untracked_cache_invalidate_path(istate, name, 0);
+	untracked_cache_invalidate_trimmed_path(istate, name, 0);
 }
 
 /*