git-am: catch missing author date early.
git-am: catch missing author date early.

Even though commit-tree would default to the current time if the incoming
e-mail message somehow did not record the timestamp, it is safer to catch
the breakage sooner.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

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