submodule update: add submodule.<name>.remote config option

submodule update: add submodule.<name>.remote config option

Don't force the user to clone from the tracked repository
(branch.<name>.remote) or `origin`.  By setting
submodule.<name>.remote in .gitmodules or the usual git config files,
you can easily point a submodule at a different remote when using
`submodule update --remote`.

The configured remote name is also used in `submodule sync` to
determine which remote.<name>.url is updated with the submodule's
origin URL.

Signed-off-by: W. Trevor King <wking@tremily.us>