worktree: add test for path handling in linked worktrees

A failure scenario reported in an earlier patch series[1] that several
`git worktree` subcommands failed or misbehaved when invoked from within
linked worktrees that used relative paths.

This adds a test that executes a `worktree prune` command inside both an
internally and an externally linked worktree and asserts that the other
worktree was not pruned.

[1]: https://lore.kernel.org/git/CAPig+cQXFy=xPVpoSq6Wq0pxMRCjS=WbkgdO+3LySPX=q0nPCw@mail.gmail.com/

Reported-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Caleb White <cdwhite3@pm.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2401-worktree-prune.sh

@@ -120,4 +120,23 @@ test_expect_success 'prune duplicate (main/linked)' '
 	! test -d .git/worktrees/wt
 '
 
+test_expect_success 'not prune proper worktrees when run inside linked worktree' '
+	test_when_finished rm -rf repo wt_ext &&
+	git init repo &&
+	(
+	    cd repo &&
+	    echo content >file &&
+	    git add file &&
+	    git commit -m msg &&
+	    git worktree add ../wt_ext &&
+	    git worktree add wt_int &&
+	    cd wt_int &&
+	    git worktree prune -v >out &&
+	    test_must_be_empty out &&
+	    cd ../../wt_ext &&
+	    git worktree prune -v >out &&
+	    test_must_be_empty out
+	)
+'
+
 test_done