New annex.synccontent config setting

New annex.synccontent config setting

.. which can be set to true to make git annex sync default to --content.

This may become the default at some point in the future.

As well as being configuable by git config, it can be configured by
git-annex config to control the default behavior in all clones of a
repository.

Had to add a separate --no-content switch to we can tell if it's been
explicitly set, and should override annex.synccontent. If --content was the
default, this complication would not be necessary.

This commit was sponsored by Jake Vosloo on Patreon.