Allow git-remote to update named groups of remotes

Allow git-remote to update named groups of remotes

In response to a feature request from Shawn Pearce, this patch allows
a user to update a named group of remotes by using "git remote update
<group>", where the group is defined in the config file by
remotes.<group>.  The default if the named group is not specified is
now fetched group remotes.default, instead of remote.fetch, which is
what had been previously used.

In addition, if remotes.default is not defined, all remotes defined in
the config file will be used, as before, but there is now also
possible to request that a particular repository to be skipped by
default by using the boolean configuration parameter
remote.<name>.skipDefaultUpdate.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>