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.

adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2