[PATCH 0 of 4 V2] largefiles: update largefiles in a subrepo when updating the parent (issue3752)

[Matt Harbison]

This is very similar to the first version, except it is now on default and I skirted Mads' objection to the override method producing output while the original doesn't by making the original produce output too.  This was a fallback- my alternate idea of going back to calling hg.clean and hg.update and his idea of overriding hgsubrepo.get() both seemed problematic.  The tests run cleanly now with this change.

Patches 3 and 4 can maybe be folded into 2, but I think they can stand on their own.