setup: set whole env for running hg even if .hg doesn't exist (issue3073)
The issue was partially fixed by b357a972d6cd, but there were missing
variables to run build_hgextindex.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.