QGit should support git worktree

[tibirna]

Submitted by Daniel Levin on 2016-01-06.

Since Git 2.5 release Git supports multiple working directories. QGit should be able to open such working copy as usual one.

This change does not require much effort as far as I see:

• Normally .git is a bare repository, in separate working tree it is a file with path to bare repository
• HEAD and ORIG_HEAD can be located not in .git/ but in original barerepository/worktrees/barerepository/worktrees/tree_name

In addition QGit should also support opening bare repository to show history without HEAD. This can be addressed in separate issue I believe.