format-patch: fix two-argument special case, and make it easier to pick single commits

Luben Tuikov noticed that sometimes being able to say
'git-format-patch <commit>' to format the change a single commit
introduces relative to its parent is handy.

This patch does not support that directly, but it makes sense to
interpret a single argument "rev" to mean "rev^1..rev".

With this, the backward compatibility syntaxes still apply:

 - "format-patch master" means "format-patch master..HEAD"
 - "format-patch origin master" means "format-patch origin..master"
 - "format-patch origin.." means "format-patch origin..HEAD"

But "format-patch a b c d e" formats the changes these five
commits introduce relative to their respective parents.  Earlier
it rejected these arguments not in "one..two" form.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-format-patch.sh

@@ -99,7 +99,7 @@ filelist=$tmp-files
 # Also, "rev1.." should mean "rev1..HEAD"; git-diff users are
 # familiar with that syntax.
 
-case "$#,$1" in
+case "$#,$1$2" in
 1,?*..?*)
 	# single "rev1..rev2"
 	;;
@@ -131,7 +131,8 @@ do
 		rev2=`expr "$revpair" : '.*\.\.\(.*\)'`
 		;;
 	*)
-		usage
+		rev1="$revpair^"
+		rev2="$revpair"
 		;;
 	esac
 	git-rev-parse --verify "$rev1^0" >/dev/null 2>&1 ||