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's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.