Use git-mailinfo to extract author informations from .topmsg

This has the benefit that the [PATCH] prefixes are stripped and so it
might help to prevent you sending mails with two prefixes.  (As happend
to me after I git-format-patch'd an exported series and sent it out with
git-send-email.)

Moreover it should make the code more robust and it allows to remove a
helper function.
And it allows you to set add a Date: line to .topmsg which is then used
as author date.

Signed-off-by: Uwe Kleine-König <ukleinek@strlen.de>