beautifygraph: don't warn about busted terminal if HGPLAIN is set
This was breaking some automation for users that discovered the
extension and turned it on.
Differential Revision: https://phab.mercurial-scm.org/D4338
merge: do not delete untracked files silently (
issue5962)
37450a122128,
2a774cae3a03, and
656ac240f392 weren't enough to prevent data
loss. No unknown "files" weren't deleted before
7a8a16f8ea22, "context: also
consider path conflicts when clearing unknown files."