Add the configuration option skipFetchAll

Add the configuration option skipFetchAll

Implement the configuration skipFetchAll option to allow
certain remotes to be skipped when doing 'git fetch --all' and
'git remote update'. The existing skipDefaultUpdate variable
is still honored (by 'git fetch --all' and 'git remote update').
(If both are set in the configuration file with different values,
the value of the last occurrence will be used.)

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