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.

%%% should show a removed and b added
A b
R a
reverting...
%%% should show b unknown and a back to normal
? b
merging a
%%% should show foo-b
foo-b
%%% should show a removed and b added
A b
R a
reverting...
%%% should show b unknown and a marked modified (merged)
? b
%%% should show foo-b
foo-b