hgweb: don't raise an exception when displying empty repos The nullid node claims it's in the default branch, but the branch dict is empty. This fixes the main symptom from issue696, but we may want to set branchtags()['default'] = nullid somewhere for empty repos.

% 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