fetch/push: fix usage strings

fetch/push: fix usage strings

 - use "<options>" instead of just "options".
 - use "[<repository> [<refspec>...]]" to indicate that <repository> and
   <refspec> are optional, and that <refspec> cannot be specified
   without specifying <repository>.

Note that when called without specifying <repository> (eg. "git fetch
-f"), it is accurate to say that the "git fetch [<options>]
[<repository> ...]" case takes precedence over "git fetch [<options>]
<group>".

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>