git-branch-list: Allow commands to work in Git worktrees

Previously, `git-branch-list` didn't work inside worktree directories.
It expected the current directory to be a descendant of the main Git
repository directory, which isn't the case for worktrees.

Given a Git repository `test-repo`:

    $ git rev-parse --show-toplevel
    /tmp/test-repo
    $ git rev-parse --git-common-dir
    .git
    $ mkdir subdir && cd subdir
    $ git rev-parse --git-common-dir
    ../.git
    $ cd -

    $ git worktree add ../test-repo-worktree
    $ cd ../test-repo-worktree
    $ git rev-parse --show-toplevel
    /tmp/test-repo-worktree
    $ git rev-parse --git-common-dir
    /tmp/test-repo/.git

The `git rev-parse --show-toplevel` command outputs the top-level path
of the worktree, not the main repository directory. Using `git rev-parse
--git-common-dir`, we can get the correct path whether we're in a
subdirectory or a worktree.

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