Allow mailsplit (and hence git-am) to handle mails with CRLF line-endings

Allow mailsplit (and hence git-am) to handle mails with CRLF line-endings

It is not that uncommon to have mails with DOS line-ending, notably
Thunderbird and web mailers like Gmail (when saving what they call
"original" message).  So modify mailsplit to convert CRLF line-endings to
just LF.

Since git-rebase is built on top of git-am, add an option to mailsplit to
be used by git-am when it is acting on behalf of git-rebase, to refrain
from doing this conversion.

And add a test to make sure that rebase still works.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>