ls-files -i: micro-optimize path_excluded()

As we know a caller that does not recurse is calling us in the index
order, we can remember the last directory we found to be excluded
and see if the path we are looking at is still inside it, in which
case we can just answer that it is excluded.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

dir.c

@@ -592,11 +592,25 @@ void path_exclude_check_clear(struct path_exclude_check *check)
 	strbuf_release(&check->path);
 }
 
+/*
+ * Is the ce->name excluded?  This is for a caller like show_files() that
+ * do not honor directory hierarchy and iterate through paths that are
+ * possibly in an ignored directory.
+ *
+ * A path to a directory known to be excluded is left in check->path to
+ * optimize for repeated checks for files in the same excluded directory.
+ */
 int path_excluded(struct path_exclude_check *check, struct cache_entry *ce)
 {
 	int i, dtype;
 	struct strbuf *path = &check->path;
 
+	if (path->len &&
+	    path->len <= ce_namelen(ce) &&
+	    !memcmp(ce->name, path->buf, path->len) &&
+	    (!ce->name[path->len] || ce->name[path->len] == '/'))
+		return 1;
+
 	strbuf_setlen(path, 0);
 	for (i = 0; ce->name[i]; i++) {
 		int ch = ce->name[i];
@@ -608,6 +622,10 @@ int path_excluded(struct path_exclude_check *check, struct cache_entry *ce)
 		}
 		strbuf_addch(path, ch);
 	}
+
+	/* An entry in the index; cannot be a directory with subentries */
+	strbuf_setlen(path, 0);
+
 	dtype = ce_to_dtype(ce);
 	return excluded(check->dir, ce->name, &dtype);
 }