Teach the --all option to 'git fetch'

Teach the --all option to 'git fetch'

'git remote' is meant for managing remotes and 'git fetch' is meant
for actually fetching data from remote repositories. Therefore, it is
not logical that you must use 'git remote update' to fetch from
more than one repository at once.

Add the --all option to 'git fetch', to tell it to attempt to fetch
from all remotes. Also, if --all is not given, the <repository>
argument is allowed to be the name of a group, to allow fetching
from all repositories in the group.

Other options except -v and -q are silently ignored.

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