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.
