... so we can clean up correctly.

Conflicts:
	src/branch.c
	tests-clar/refs/branches/create.c

Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.

This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.

Fixes #1096

This fixes some various warnings that showed up in Travis and
a couple uses of uninitialized memory and one memory leak.

This is a major reworking of checkout strategy options.  The
checkout code is now sensitive to the contents of the HEAD tree
and the new options allow you to update the working tree so that
it will match the index content only when it previously matched
the contents of the HEAD.  This allows you to, for example, to
distinguish between removing files that are in the HEAD but not
in the index, vs just removing all untracked files.

Because of various corner cases that arise, etc., this required
some additional capabilities in rmdir and other utility functions.

This includes the beginnings of an implementation of code to read
a partial tree into the index based on a pathspec, but that is
not enabled because of the possibility of creating conflicting
index entries.

Pedantic ordering of GIT_UNUSED vs. variable declarations.

git_indexer_stats and friends -> git_transfer_progress*

Also made git_transfer_progress members more sanely
named.

Also implemented in the git2 example.

Split the clone tests into network and no-network suites.

The old method was avoiding re-loading of packfiles by watching the mtime of the
pack directory. This causes the ODB to become stale if the directory and packfile
are written within the same clock millisecond, as when cloning a fairly small
repo.

This method tries to find the object in the cached packs, and forces a refresh when
that fails. This will cause extra stat'ing on a miss, but speeds up the success
case and avoids this race condition.

* Removed the #define for defaults
* Promoted progress structure to top-level API call
  argument

Refactor checkout into several more-sensible
entry points, which consolidates common options
into a single structure that may be passed around.

Signed-off-by: Ben Straub <bstraub@github.com>