Update some ancient "set X=Y" environment code for new Windows execution without a shell

Summary:
Depends on D21051. Ref T13504. Ref T13209. This very old Mercurial code uses "X=Y hg ..." on Linux and "set X=Y & hg ..." on Windows.

The latter construct no longer works because we bypass the shell. The former construct is obsolete.

Additionaly, delete some ancient "branch merge" code which has no callers.

Test Plan: Created a diff in a Mercurial repository on Linux. I minimally vetted this on Windows since I don't have a "hg + Windows" environment at the moment.

Maniphest Tasks: T13504, T13209

Differential Revision: https://secure.phabricator.com/D21052