save settings from untrusted config files in a separate configparser
This untrusted configparser is a superset of the trusted configparser,
so that interpolation still works.
Also add an "untrusted" argument to ui.config* to allow querying
ui.ucdata.
With --debug, we print a warning when we read an untrusted config
file, and when we try to access a trusted setting that has one value
in the trusted configparser and another in the untrusted configparser.
% init
% commit
adding base
% qnew mqbase
% qrefresh
% qdiff
diff -r 67e992f2c4f3 base
--- a/base
+++ b/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qdiff dirname
diff -r 67e992f2c4f3 base
--- a/base
+++ b/base
@@ -1,1 +1,1 @@ base
-base
+patched