Convert vendor imports to git instructions

Now that subversion is gone, replace the subversion info with git info.

Note: This was pulled in from my github repo. Fixes contributed
by jrm@, uqs@, woodsb02@, delphij@, rpokala@ and ceri@

Reviewed by: emaste@ (earlier version, conditional), gjb@
Differential Revision: https://reviews.freebsd.org/D28474