format-patch: document and test --reroll-count

Signed-off-by: Junio C Hamano <gitster@pobox.com>
format-patch: give --reroll-count a short synonym -v

Accept "-v" as a synonym to "--reroll-count", so that users can say
"git format-patch -v4 master", instead of having to fully spell it
out as "git format-patch --reroll-count=4 master".

As I do not think of a reason why users would want to tell the
command to be "verbose", I think this should be OK.

Signed-off-by: Junio C Hamano <gitster@pobox.com>