Add API to calculate ahead/behind count

Fix a typo that caused a failing test, and use cl_assert_equal
instead of cl_assert.

`revwalk.h:commit_lookup()` -> `git_revwalk__commit_lookup()`
and make `git_commit_list_parse()` do real error checking that
the item in the list is an actual commit object.  Also fixed an
apparent typo in a test name.

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

Moved it into graph.{c,h} which i created for the new "graph"
functions namespace. Also adjusted the function prototype
to use `size_t` and `const git_oid *`.

Adds a repo with a more complex topology to test the ahead-behind
count.

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).