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.

1 files changed, 36 insertions, 0 deletions

diff --git a/t/108-works-with-git-worktrees.t b/t/108-works-with-git-worktrees.t
new file mode 100644
index 0000000..b2901c0
--- /dev/null
+++ b/t/108-works-with-git-worktrees.t
@@ -0,0 +1,36 @@
+#!/usr/bin/env perl -w
+
+use strict;
+
+use Test::More;
+use File::Path qw(remove_tree);
+
+use Bin qw($BIN);
+
+chdir 't-git-repo' or die $!;
+
+system('git branch a-branch');
+ok !$?;
+
+system("$BIN save a-branch");
+ok !$?;
+
+system('git worktree add ../t-git-repo-worktree');
+ok !$?;
+
+chdir '../t-git-repo-worktree' or die $!;
+
+my $branch_list = qx($BIN);
+is $branch_list, '     1	a-branch
+';
+
+
+# Teardown
+system("$BIN clear");
+chdir '../t-git-repo' or die $!;
+remove_tree('../t-git-repo-worktree') or die $!;
+system('git checkout master');
+system('git branch -d a-branch');
+
+
+done_testing;