Fix bad behaviour when specifying an invalid date (
issue700)
commit (aborts _after_ typing in a commit message)
backout (aborted after the initial revert)
tag (edited .hgtags and couldn't commit)
import (patch applied, then commit fails)
qnew (aborts on bad dates, but writes any valid date into the # Date header)
qrefresh (like qnew)
sign (like tag)
fetch (merge, merge, merge, merge, abort)