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.