Request the addition of a repo-plugin flag for setting the "--force-sync" option on repo sync.

See discussions in https://groups.google.com/forum/#!searchin/repo-discuss/force$20sync/repo-discuss/wq9_C6gj1bk/dDNyHGLpAAAJ about the necessity of using the --force-sync option in some situations, especially when git projects are removed in the manifest.