clang-SVN HEAD kindly provided my the info, that sm_repo maybe
uninitialized when we want to free it (If the expression in line 358 or
359/360 evaluate to true, we jump to "cleanup", where we'd use sm_repo
uninitialized).

Loads a disk alternate by path to the ODB. Mimics the
`GIT_ALTERNATE_OBJECT_DIRECTORIES` shell var.

The Great Renaming of 2012

This makes the first OID param of the ODB callback a const pointer
and also propogates that change all the way to the backends.

Includes typedef for git_direction, and renames for
GIT_DIR_[FETCH|PUSH] to GIT_DIRECTION_(\1).

In so doing, promote commit_list to git_commit_list,
with its own internal API header.

This fixes some missed places where we can apply const-ness to
various public APIs.

There are still some index and tree APIs that cannot take const
pointers because we sort our `git_vectors` lazily and so we can't
reliably bsearch the index and tree content without applying a
`git_vector_sort()` first.

This also fixes some missed places where size_t can be used and
where const can be applied to a couple internal functions.

This makes the diff functions that take callbacks both take
the payload parameter after the callback function pointers and
pass the payload as the last argument to the callback function
instead of the first.  This should make them consistent with
other callbacks across the API.

As part of API review, use a typedef for the callback fn ptr.