Document reparse --min-date more, add validation

Summary:
Sometimes it seems necessary to force a reparse of recent commits in
production, it took me longer than expected to get this right.

To make this easier, document the usage of --min-date
further with usage examples and print a usage exception with the input
if the supplied value isn't accepted by MySQL.
(otherwise all commits will be affected in the case of user error)

Test Plan:
.. create TEST repo with commits dated 2013-04-03 ..

$ ./reparse.php --all TEST --owners --min-date "2013a-04-03 10:30:19"
.. see usage exception - invalid timestamp ..

$ ./reparse.php --all TEST --owners --min-date "2013-04-03 10:30:19"
.. reparse commits ok ..

$ ./reparse.php --all TEST --owners --min-date "2013-04-04 10:30:19"
.. see 'No commits have been discovered' ..

$ ./reparse.php --all TEST --owners
.. reparse commits ok ..

$ ./reparse.php --help
.. looks ok to me ..

$ ./reparse.php --all TEST --owners --min-date 2013-04-03 10:30:19
.. see error - interprets 10:30:19 as commit and refuses ..

$ ./reparse.php --all TEST --owners --min-date <<first commit time>>
.. parse this commit and following ..

$ ./reparse.php <<revision_id>> --owners --min-date <<first commit time>>
.. see error - insist on --all if --min-date ..

$ ./reparse.php <<revision_id>> --owners
.. ok ..

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5579