Implement --newrepo flag.

The --newrepo flag tells emerge to recompile a package if it is now
being pulled from a different repository.

BUG=chromium:200417
TEST=Verify ebuilds get pulled in when repo changes.
TEST=New test cases.
Change-Id: I8d5ff1cfc8c1d23e00c733b861547d3626bc2ece