git-am: Add command line parameter `--keep-cr` passing it to git-mailsplit

git-am: Add command line parameter `--keep-cr` passing it to git-mailsplit

c2ca1d7 (Allow mailsplit (and hence git-am) to handle mails with CRLF
line-endings, 2009-08-04) fixed "git mailsplit" to help people with
MUA whose output from save-as command uses CRLF as line terminators by
stripping CR at the end of lines.

However, when you know you are feeding output from "git format-patch"
directly to "git am", and especially when your contents have CR at the
end of line, such stripping is undesirable.  To help such a use case,
teach --keep-cr option to "git am" and pass that to "git mailinfo".

Signed-off-by: Stefan-W. Hahn <stefan.hahn@s-hahn.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>