[PATCH 0 of 5] Add 'unified' tests to the test framework

[Matt Mackall]

This is an experiment in unifying test commands and results as
discussed at the 1.5 sprint. See the last three patches for what tests
look like in this model. It's quite an improvement. And when something
breaks, you get to see the changed output in the context of the
commands that caused it. Also, there's some basic support for
filtering of output with regexes, like this:

Strip 1: expose an old head:

  $ hg --config extensions.mq= strip 5
  saved backup bundle to .*

..which means a lot of the hoops tests jump through right now with sed
and grep are no longer needed.