I accidentally wrote a separate priority queue implementation when
I was working on file rename detection as part of the file hash
signature calculation code. To simplify licensing terms, I just
adapted that to a general purpose priority queue and replace the
old priority queue implementation that was borrowed from elsewhere.
This also removes parts of the COPYING document that no longer
apply to libgit2.
`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.
