view CONTRIBUTING @ 33885:800bb35d891e

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

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