Fix uninitialized variable

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.