send-email: add option -h
send-email: add option -h

Most other git commands print a synopsis when passed -h. Make
send-email do the same.

Signed-off-by: Clemens Buchacher <3561697699be488d2983f82b126a1881ffc3afbc@aon.at>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git