Parse commit time as uint64_t to avoid overflow

The commit time is already stored as a git_time_t, but we were
parsing is as a uint32_t.  This just switches the parser to use
uint64_t which will handle dates further in the future (and adds
some tests of those future dates).
Merge pull request #1265 from arrbee/parse-commit-time-as-uint64

Parse commit time as uint64_t to avoid overflow