add HGRCPATH env var, list of places to look for hgrc files.
if set, override default hgrc search path.
if empty, only .hg/hgrc of current repo read.
for each element, if directory, all entries in directory with end in
".rc" are added to path. else, element is added to path.
big thing about this change is that user "~/.hgrc" and system hgrc not
longer breaks tests. run-tests makes HGRCPATH empty now.
#!/bin/sh
hg init 1
echo '[ui]' >> 1/.hg/hgrc
echo 'timeout = 10' >> 1/.hg/hgrc
echo foo > 1/foo
hg --cwd 1 ci -A -m foo
hg clone 1 2
hg clone 2 3
echo '[hooks]' >> 2/.hg/hgrc
echo 'changegroup.push = hg push -qf ../1' >> 2/.hg/hgrc
echo bar >> 3/foo
hg --cwd 3 ci -m bar
hg --cwd 3 push ../2