git am/mailinfo: Don't look at in-body headers when rebasing

git am/mailinfo: Don't look at in-body headers when rebasing

When we are rebasing we know that the header lines in the
patch are good and that we don't need to pick up any headers
from the body of the patch.

This makes it possible to rebase commits whose commit message
start with "From" or "Date".

Test vectors by Jeff King.

Signed-off-by: Lukas Sandström <luksan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>