git-worktree implementation

[nelsonni]

Is your feature request related to a problem? Please describe.
git-worktree provides the ability to manage multiple working trees:

A git repository can support multiple working trees, allowing you to check out more than one branch at a time. With git worktree add a new working tree is associated with the repository. This new working tree is called a "linked working tree" as opposed to the "main working tree" prepared by git-init[1] or git-clone[1]. A repository has one main working tree (if it’s not a bare repository) and zero or more linked working trees. When you are done with a linked working tree, remove it with git worktree remove.

In its simplest form, git worktree add automatically creates a new branch whose name is the final component of , which is convenient if you plan to work on a new topic. For instance, git worktree add ../hotfix creates new branch hotfix and checks it out at path ../hotfix. To instead work on an existing branch in a new working tree, use git worktree add . On the other hand, if you just plan to make some experimental changes or do testing without disturbing existing development, it is often convenient to create a throwaway working tree not associated with any branch. For instance, git worktree add -d creates a new working tree with a detached HEAD at the same commit as the current branch.

Describe the solution you'd like
Implementing git-worktree within isomoprhic-git would be ideal, but until then we can wrap the functionality of git-worktree around the necessary functions exposed by isomorphic-git. The following functions must be implemented:

• git-worktree.add (added in 3e66b8e)
• git-worktree.list (added in abfd61e)
• git-worktree.lock
• git-worktree.move
• git-worktree.prune (added in 67e7455)
• git-worktree.remove (added in 15aa493)
• git-worktree.repair
• git-worktree.unlock

Not included in the git-worktree API, but necessary for all other interactions:

• git-worktree.status (added in 61fea88)
• git-worktree.statusMatrix (added in fd51347)

Describe alternatives you've considered
N/A

Additional context
All src/containers/git.ts functions that interact with files will probably need to be updated to support the different path structures in git-worktree directories.