packaging: don't crash building wix with python3.6 and earlier `capture_output` was added in 3.7. I was tempted to just check and abort in build.py, since Windows doesn't have the Linux problem where some distros only ship an older python. But this is in a library that could be used elsewhere in the future.

chistedit: add basic colours to diff view This isn't complete, and it would be nice to show the exact same colours that `hg diff` would show. That goal is too lofty, so this just shows some basic colours, on the premise that a little is better than nothing.