While we already provide functionality to look up a worktree from a
repository, we cannot do so the other way round. That is given a
repository, we want to look up its worktree if it actually exists.
Getting the worktree of a repository is useful when we want to get
certain meta information like the parent's location, getting the locked
status, etc.
Implement `git_repository_head_for_worktree` and
`git_repository_head_detached_for_worktree` for directly accessing a
worktree's HEAD without opening it as a `git_repository` first.
Implement the `git_worktree_prune` function. This function can be
used to delete working trees from a repository. According to the
flags passed to it, it can either delete the working tree's
gitdir only or both gitdir and the working directory.
Working trees support locking by creating a file `locked` inside
the tree's gitdir with an optional reason inside. Support this
feature by adding functions to get and set the locking status.
Add a new function that checks wether a given `struct
git_worktree` is valid. The validation includes checking if the
gitdir, parent directory and common directory are present.
Introduce a new `struct git_worktree`, which holds information
about a possible working tree connected to a repository.
Introduce functions to allow opening working trees for a
repository.
