Add support for worktrees in `fromExisting`

[dnys1]

Adds support for worktree paths in GitDir.fromExisting. Worktrees are identified based on whether the $GIT_DIR and $GIT_COMMON_DIR are different (although not sure there are any edge cases worth considering here).

The logic is refactored a bit to make clear that the only times when GitDir.fromExisting throws are in the cases where:

• gitDirRoot is not a git directory or worktree
• gitDirRoot is a subdirectory of a git directory or worktree and allowSubdirectory is false.