worktree: add test for path handling in linked worktrees

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>