perf: make the tests work in worktrees

This patch makes perf-lib.sh more robust so that it can run correctly
even inside a worktree. For example, it assumed that $GIT_DIR/objects is
the objects directory (which is not the case for worktrees) and it used
the commondir file verbatim, even if it contained a relative path.

Furthermore, the setup code expected `git rev-parse --git-dir` to spit
out a relative path, which is also not true for worktrees. Let's just
change the code to accept both relative and absolute paths, by avoiding
the `cd` into the copied working directory.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/perf-lib.sh

@@ -80,22 +80,22 @@ test_perf_create_repo_from () {
 	error "bug in the test script: not 2 parameters to test-create-repo"
 	repo="$1"
 	source="$2"
-	source_git=$source/$(cd "$source" && git rev-parse --git-dir)
+	source_git="$(git -C "$source" rev-parse --git-dir)"
+	objects_dir="$(git -C "$source" rev-parse --git-path objects)"
 	mkdir -p "$repo/.git"
 	(
-		cd "$repo/.git" &&
-		{ cp -Rl "$source_git/objects" . 2>/dev/null ||
-			cp -R "$source_git/objects" .; } &&
+		{ cp -Rl "$objects_dir" "$repo/.git/" 2>/dev/null ||
+			cp -R "$objects_dir" "$repo/.git/"; } &&
 		for stuff in "$source_git"/*; do
 			case "$stuff" in
-				*/objects|*/hooks|*/config)
+				*/objects|*/hooks|*/config|*/commondir)
 					;;
 				*)
-					cp -R "$stuff" . || exit 1
+					cp -R "$stuff" "$repo/.git/" || exit 1
 					;;
 			esac
 		done &&
-		cd .. &&
+		cd "$repo" &&
 		git init -q && {
 			test_have_prereq SYMLINKS ||
 			git config core.symlinks false