Add --signoff and --verify option to git commit.

Add --signoff and --verify option to git commit.

As brought up in the discussion which followed a patch to add a
signed-off-by line with the --sign flag to format-patch from
Johannes Schindelin, add --signoff to the git commit command.

Also add --verify to make sure the lines you introduced are
clean, which is more useful in commit but not very much in
format-patch as it was originally implemented, because finding
botches at format-patch time is too late.

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