subrepo: rename the '_ui' member to 'ui'
The localrepository class has a 'ui' member, so keeping the names the same will
allow for duck typing with subrepo instances when accessing 'ui'. Changing this
is easier than finding all of the localrepository instance uses and renaming
that to '_ui'.
% creating repo
% add and commit
C test.py
% change
M test.py