We moved the "main" parsing to use 64 bits for the timestamp, but the
quick parsing for the revwalk did not. This means that for large
timestamps we fail to parse the time and thus the walk.

Move this parser to use 64 bits as well.

FLAG_BITS only seems to be used internally

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.

In so doing, promote commit_list to git_commit_list,
with its own internal API header.