update-index --again: take optional pathspecs

When pathspecs are given, update-index --again further limits
the set of paths to be updated to those that match them.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/t2101-update-index-reupdate.sh

@@ -69,5 +69,14 @@ test_expect_success 'update-index --update from subdir' \
 100644 0f1ae1422c2bf43f117d3dbd715c988a9ed2103f 0	file2
 EOF'
 
-test_done
+test_expect_success 'update-index --update with pathspec' \
+	'echo very happy >file2 &&
+	cat file2 >dir1/file3 &&
+	git-update-index --again dir1/ &&
+	git-ls-files -s >current &&
+	cmp current - <<\EOF
+100644 594fb5bb1759d90998e2bf2a38261ae8e243c760 0	dir1/file3
+100644 0f1ae1422c2bf43f117d3dbd715c988a9ed2103f 0	file2
+EOF'
 
+test_done

update-index.c

@@ -610,6 +610,7 @@ static int do_reupdate(int ac, const char **av,
 	 */
 	int pos;
 	int has_head = 1;
+	char **pathspec = get_pathspec(prefix, av + 1);
 
 	if (read_ref(git_path("HEAD"), head_sha1))
 		/* If there is no HEAD, that means it is an initial
@@ -621,7 +622,8 @@ static int do_reupdate(int ac, const char **av,
 		struct cache_entry *ce = active_cache[pos];
 		struct cache_entry *old = NULL;
 		int save_nr;
-		if (ce_stage(ce))
+
+		if (ce_stage(ce) || !ce_path_match(ce, pathspec))
 			continue;
 		if (has_head)
 			old = read_one_ent(NULL, head_sha1,