worktree_dir isn't validated when it should be

In case where a branch is getting renamed, all HEADs of the main
repository and of its worktrees that point to the old branch need to get
updated to point to the new branch. We already do so and have a test for
this, but the test only verifies that we're able to lookup the updated
HEAD, not what it contains.

Let's make the test more specific by verifying the updated HEAD also has
the correct updated symbolic target.

The function `git_repository_head_for_worktree` currently uses
`git_reference__read_head` to directly read a given worktree's HEAD from
the filesystem. This is broken in case the repository uses a different
refdb implementation than the filesystem-based one, so let's instead
open the worktree as a real repository and use `git_reference_lookup`.
This also fixes the case where the worktree's HEAD is not a symref, but
a detached HEAD, which would have resulted in an error previously.

The function `git_repository_foreach_head` is broken, as it directly
interacts with the on-disk representation of the reference database,
thus assuming that no other refdb is used for the given repository. As
this is an internal function only and all users have been replaced,
let's remove this function.

Given a Git repository, it's non-trivial to iterate over all worktrees
that are associated with it, including the "main" repository. This
commit adds a new internal function `git_repository_foreach_worktree`
that does this for us.

We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.