Support using a linked worktree as a local content source

A local git repository can be configured to have multiple worktrees (referred to as linked worktrees). The main worktree works in the normal way, containing a .git foler. Linked worktrees (which are located elsewhere) each have a .git file instead of a .git folder. The .git file contains the path to the location of the actual .git folder (the gitdir).

Here's an example of the contents of a .git file in a linked worktree:

gitdir: /home/usr/projects/project-name/master/.git/worktrees/v2.2.x

All Antora needs to do is retrieve the gitdir path from this file and pass it to isomorphic git. The rest will work as is.