pushvars: do not mangle repo state Setting `repo._shellvars` works but is not a clean way to pass the pushvars information from the push command to the exchange operation. Therefore change it to actually pass `pushvars` as a push operation argument instead. This makes third party extension like remotenames easier to support pushvars cleanly. The key value parsing and verification code has been moved to a lower level so it's harder to be bypassed and easier to be used in remotenames which could replace `push` command entirely. Differential Revision: https://phab.mercurial-scm.org/D423