git-format-patch: add --output-directory long option again

Additionally notices and complains to an -o option without
directory or a duplicated -o option, -o and --stdout given
together.  Also delays the creation of directory until all
arguments are parsed, so that the command does not leave an
empty directory behind when it exits after seeing an unrelated
invalid option.

[jc: originally from Dennis Stosberg but with minor fixes, and
 documentation updates from Dennis.]

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