Sort commits by date in descending order

Commit day could be in the past compared to push day (e.g. when rebase
took place). This leads to invalid order of days in /commits and
/commits/search routes. This patch sorts categorized commits so that order
is always descending.
Change date format for correct ordering
Merge pull request #457 from mikedld/sort-commits-by-date

Sort commits by date in descending order