fsck.c:fsck_ident(): ident points at a const string

Since fsck_ident doesn't change the content of **ident, the type of
ident could be const char **.

This change is required to rewrite fsck_commit() to use skip_prefix().

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -243,7 +243,7 @@ static int fsck_tree(struct tree *item, int strict, fsck_error error_func)
 	return retval;
 }
 
-static int fsck_ident(char **ident, struct object *obj, fsck_error error_func)
+static int fsck_ident(const char **ident, struct object *obj, fsck_error error_func)
 {
 	if (**ident == '<')
 		return error_func(obj, FSCK_ERROR, "invalid author/committer line - missing space before email");
@@ -281,7 +281,7 @@ static int fsck_ident(char **ident, struct object *obj, fsck_error error_func)
 
 static int fsck_commit(struct commit *commit, fsck_error error_func)
 {
-	char *buffer = commit->buffer;
+	const char *buffer = commit->buffer;
 	unsigned char tree_sha1[20], sha1[20];
 	struct commit_graft *graft;
 	int parents = 0;