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'.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a