Feature request: support git worktrees

[lubieowoce]

Currently, if I try to use this extension in a worktree, it will error with

.git/HEAD does not exist

This is the same error as #23 , but it's happening for a different reason, so I thought it makes sense to open a separate issue.

The problem is that, in a git worktree, .git is a text file like this:

gitdir: <path to base repo>

so any code that assumes that .git is a directory will break.

Reproduction steps

mkdir worktree-test
cd worktree-test
# create 'worktree-test/one'
git clone git@github.com:lubieowoce/copy-github-url-test-repo.git one
cd one
# create 'worktree-test/two'
git worktree add -b test-branch ../two

in one, "copy github url" will work as expected. in two, it will show an error instead.