1. internal iterators now return GIT_ITEROVER when you go past the
last item in the iteration.
2. git_iterator_advance will "advance" to the first item in the
iteration if it is called immediately after creating the
iterator, which allows a simpler idiom for basic iteration.
3. if git_iterator_advance encounters an error reading data (e.g.
a missing tree or an unreadable file), it returns the error
but also attempts to advance past the invalid data to prevent
an infinite loop.
Updated all tests and internal usage of iterators to account for
these new behaviors.
`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.
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 *`.
