Add git 2.5 `worktree` command support [This issue should be moved to SourceTree for Windows]

Git 2.5 come with a new command worktree:
https://git-scm.com/docs/git-worktree

You can use it to manage multiple working trees attached to the same repository.
It is very useful when working on large project with multiple working branches.
Hope GitUp can support this wonderful new feature.

There are some discussion on other git client libraries introducing worktree support:

• https://github.com/github/hub/pull/970
• https://youtrack.jetbrains.com/issue/IDEA-143404