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

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>