git-am support for naked email messages (take 2)

This allows git-am to accept single-message files as well as mboxes.
Unlike the previous version, this one doesn't need to be explicitly told
which one it is; rather, it looks to see if the first line is a From
line and uses it to select mbox mode or not.

I moved the logic to do all this into git-mailsplit, which got a new
user interface as result, although the old interface is still available
for backwards compatibility.

[jc: applied with two obvious fixes.]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-am.sh

@@ -164,10 +164,7 @@ else
 	# Start afresh.
 	mkdir -p "$dotest" || exit
 
-	# cat does the right thing for us, including '-' to mean
-	# standard input.
-	cat "$@" |
-	git-mailsplit -d$prec "$dotest/" >"$dotest/last" || {
+	git-mailsplit -d"$prec" -o"$dotest" -b -- "$@" > "$dotest/last" ||  {
 		rm -fr "$dotest"
 		exit 1
 	}