odb-pack: resurrect pack_backend__read_header

EINVALIDSPEC

To paraphrase @peff:

You can get both size and type from a packed object reasonably cheaply.
If you have:

* An object that is not a delta; both type and size are available in the
  packfile header.
* An object that is a delta. The packfile type will be OBJ_*_DELTA, and
  you have to resolve back to the base to find the real type. That means
  potentially a lot of packfile index lookups, but each one is
  relatively cheap. For the size, you inflate the first few bytes of the
  delta, whose header will tell you the resulting size of applying the
  delta to the base.

For simplicity, we just decompress the whole delta for now.

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 *`.