Merge branch 'js/perf-rebase-i'

The one in 'master' has a brown-paper-bag bug that breaks the perf
test when used inside a usual Git repository with a working tree.

* js/perf-rebase-i:
  perf: make the tests work without a worktree

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 ||