Conflicts:
	src/revparse.c

libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

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.

The code worked under the assumption that anything under `refs/tags` are
tag objects, and all the rest would be peelable to a commit. As it is
completely valid to have tags to blobs under a non `refs/tags` ref, this
would cause failures when trying to peel a tag to a commit.

Fix the broken filtering by switching to `git_revwalk_push_glob`, which
already handles this case.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Update internal usage to use the `git_reference` names for constants.

Two tests in network::fetchlocal explicitly set a cleanup function to
free and remove the created sandbox repositories. This is not necessary,
though, as the cleanup function executed after each test already takes
care of cleaning up after them. Remove the code to avoid needless code
duplication.

While we download the remote's remote-tracking branches, we don't
download the tag. This points to the tag auto-follow rules interfering
with the refspec.