Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

This returns the integer-cast truth value comparing the dates. What we
want instead of a (-1, 0, 1) output depending on how they compare.

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.

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.

git-core prefers younger merge bases over older ones in case that multiple valid merge bases exists.

This uses the odb object accessors so we can change the internals
more easily...