Update git_repository_open

The cleanup needs to happen anyway, so set the error code and jump
there instead of copying the code.

Plug a few leaks

As parents are older than their children, we're appending to the
commit list most of the time, which makes an ordered linked list quite
inefficient.

While we're there, don't sort the results list in the main loop, as
we're sorting them afterwards and it creates extra work.

Nothing should be hidden and this shouldn't bother the merge base
calculation.

There is no need walk down the parents of a merge base to mark them as
uninteresting because we'll never see them. Calculate the merge bases
in prepare_walk() so mark_uninteresting() can stop at a merge base
instead of walking all the way to the root.

It's implemented in revwalk.c so it has access to the revision
walker's commit cache and related functions. The algorithm is the one
used by git, modified so it fits better with the library's functions.

The code was already there, so factor it out and let users push an OID
by giving it a reference name. Only refs to commits are
supported. Annotated tags will throw an error.

Add a new command `git_repository_open_ext` with extended options
that control how searching for a repository will be done.  The
existing `git_repository_open` and `git_repository_discover` are
reimplemented on top of it.  We may want to change the default
behavior of `git_repository_open` but this commit does not do that.

Improve support for "gitdir" files where the work dir is separate
from the repo and support for the "separate-git-dir" config.  Also,
add support for opening repos created with `git-new-workdir` script
(although I have only confirmed that they can be opened, not that
all functions work correctly).

There are also a few minor changes that came up:

- Fix `git_path_prettify` to allow in-place prettifying.

- Fix `git_path_root` to support backslashes on Win32.  This fix
  should help many repo open/discover scenarios - it is the one
  function called when opening before prettifying the path.

- Tweak `git_config_get_string` to set the "out" pointer to NULL
  if the config value is not found.  Allows some other cleanup.

- Fix a couple places that should have been calling
  `git_repository_config__weakptr` and were not.

- Fix `cl_git_sandbox_init` clar helper to support bare repos.

This is Git yo. You can fetch stuff from the history if you need it.

Basic branch management API

Fix compilation warning