Push! By schu, phkelley, and congyiwu

Ignore invalid entries by default

Fix git_branch_tracking() for branches with empty merge and/or remote config entries

fetch: enhance test coverage

Enhance diff test coverage between a null Tree and the Workdir

Consolidate text buffer functions

There are many scattered functions that look into the contents of
buffers to do various text manipulations (such as escaping or
unescaping data, calculating text stats, guessing if content is
binary, etc).  This groups all those functions together into a
new file and converts the code to use that.

This has two enhancements to existing functionality.  The old
text stats function is significantly rewritten and the BOM
detection code was extended (although largely we can't deal with
anything other than a UTF8 BOM).

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).