rebase: improve error message when upstream argument is missing

rebase: improve error message when upstream argument is missing

Strip out options before checking for a missing upstream argument.
Before:

 $ git rebase -m
 shift: 426: can't shift that many

After:

 $ git rebase -m
 Usage: git rebase ...

While at it, fix the usage message to explain that the upstream
argument is mandatory.

Reported-by: Jon Dowland <jmtd@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>