How does TERM=dumb support work?

[Carsten Mattner]

When I compare the progress bar functionality of a clone
operation in hg and git running this in 9term (plan9[port])
only hg manages to do something acceptable. I searched
the archives and found patches for S390 which look like
the motivation for this.

I want to bring this up on the git mailing list but would like
to know beforehand what and how hg does manage to support
TERM=dumb without degrading too much. Knowing how
hg does it is useful to suggest or implement changes to git.