Update "convert-cache" to handle git itself.
Update "convert-cache" to handle git itself.

The git archives have some old-date-format commits with timezones
that the converter didn't recognize. Also, make it be quiet about
already-converted dates.

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git