Teach the --multiple option to 'git fetch'

Teach the --multiple option to 'git fetch'

Add the --multiple option to specify that all arguments are either
groups or remotes. The primary reason for adding this option is
to allow us to re-implement 'git remote update' using fetch.

It would have been nice if this option was not needed, but since
the colon in a refspec is optional, it is in general not possible
to know whether a single, colon-less argument is a remote or a
refspec.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>