R1:875455ad6415

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.