submodule sync: add --local and --no-local

submodule sync: add --local and --no-local

These control whether or not the submodule.<name>.url setting is set
in the superproject config.  With --local, the setting is always set.
With --no-local, it is only set if another value is already set.  Use

  $ git submodule sync --local

to forcibly populate the local config, and

  $ git submodule sync --no-local

to only update the existing local configuration.

Default to --local for compatibility with earlier versions of Git.

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