Most of our Mercurial code assumes that the command we invoked has gone

right and that the outputted results are suitable for whatever we

queried.
This isn't always the case. If configuration is wrong (say, an extension

failed to load), any usable output could be prefixed with something like:

*** failed to import extension hgsubversion: No module named 'hgsubversion'


This went to stderr, but we redirected all errors by default to stdout.

That meant that we'd end up with a lot of bad output, which wasn't

necessarily caught until piped to another Mercurial command.
We now default any execute() calls to keeping error output on its own

stream and out of our standard output. There are some commands that do

need to check error output (such as the hgsubversion checks), so they go

through the normal behavior, but the rest will only capture usable

standard output now.
In the process, most calls to execute() have been changed to

self._execute(). That function itself has had a small logic tweak to

avoid a potentially unnecessary hg call for a capability check.

All unit tests passed in my environment with the error listed in the

description.