worktree: add ability to create worktree with pre-existing branch

Currently, we always create a new branch after the new worktree's name
when creating a worktree. In some workflows, though, the caller may want
to check out an already existing reference instead of creating a new
one, which is impossible to do right now.

Add a new option `ref` to the options structure for adding worktrees. In
case it is set, a branch and not already checked out by another
worktree, we will re-use this reference instead of creating a new one.

The current signature of `git_worktree_prune` accepts a flags field to
alter its behavior. This is not as flexible as we'd like it to be when
we want to enable passing additional options in the future. As the
function has not been part of any release yet, we are still free to
alter its current signature. This commit does so by using our usual
pattern of an options structure, which is easily extendable without
breaking the API.

When creating a new worktree, we do have a potential race with us
creating the worktree and another process trying to delete the same
worktree as it is being created. As such, the upstream git project has
introduced a flag `git worktree add --locked`, which will cause the
newly created worktree to be locked immediately after its creation. This
mitigates the race condition.

We want to be able to mirror the same behavior. As such, a new flag
`locked` is added to the options structure of `git_worktree_add` which
allows the user to enable this behavior.

The `git_worktree_add` function currently accepts only a path and name
for the new work tree. As we may want to expand these parameters in
future versions without adding additional parameters to the function for
every option, this commit introduces our typical pattern of an options
struct. Right now, this structure is still empty, which will change with
the next commit.

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.