Fix git-worktree support
Use git-show-ref instead of git-ls-remote to properly list references in
worktrees. The latter was failing with:
$ git ls-remote /opt/tig/test/tmp/status/worktree-test/repo/.git/worktrees/work dir
fatal: '/opt/tig/test/tmp/status/worktree-test/repo/.git/worktrees/work dir' does not appear to be a git repository
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists
Initial test case provided by stenja.
Fixes #437