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 a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository 'subrepo'
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