Add worktrees read support

Add worktrees read support

Based on deritative work done in Andre's work in [1].

This change focuses on adding support for reading the repository
state when branches are checked out using git's worktrees.

I've refactored original work by removing all unrelevant
changes which were mostly around refactoring to extract
i.e. constants which mostly created noise for a review.

I've tried to address original review comments:
- Not adding non-behavioral changes
- "HEAD" should get resolved from gitDir
- Reftable recently landed in cgit 2.45,
  see https://github.com/git/git/blob/master/Documentation/RelNotes/2.45.0.txt#L8
  We can add worktree support for reftable in a later change.
- Some new tests to read from a linked worktree which
  is created manually as there's no write support.

[1] https://git.eclipse.org/r/c/jgit/jgit/+/163940/18

Change-Id: Id077d58fb6c09ecb090eb09d5dbc7edc351a581d