perf: make the tests work without a worktree

In regular repositories $source_git and $objects_dir contain relative
paths based on $source.  Go there to allow cp to resolve them.
Signed-off-by: NRene Scharfe <l.s.r@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/perf/perf-lib.sh

@@ -84,6 +84,7 @@ test_perf_create_repo_from () {
 	objects_dir="$(git -C "$source" rev-parse --git-path objects)"
 	mkdir -p "$repo/.git"
 	(
+		cd "$source" &&
 		{ cp -Rl "$objects_dir" "$repo/.git/" 2>/dev/null ||
 			cp -R "$objects_dir" "$repo/.git/"; } &&
 		for stuff in "$source_git"/*; do
@@ -94,7 +95,9 @@ test_perf_create_repo_from () {
 					cp -R "$stuff" "$repo/.git/" || exit 1
 					;;
 			esac
-		done &&
+		done
+	) &&
+	(
 		cd "$repo" &&
 		git init -q && {
 			test_have_prereq SYMLINKS ||