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>

git-am.sh

@@ -197,7 +197,13 @@ check_patch_format () {
 split_patches () {
 	case "$patch_format" in
 	mbox)
-		git mailsplit -d"$prec" -o"$dotest" -b -- "$@" > "$dotest/last" ||
+		case "$rebasing" in
+		'')
+			keep_cr= ;;
+		?*)
+			keep_cr=--keep-cr ;;
+		esac
+		git mailsplit -d"$prec" -o"$dotest" -b $keep_cr -- "$@" > "$dotest/last" ||
 		clean_abort
 		;;
 	stgit-series)