help: distinguish sections when multiple match (issue4802)

merge with stable

hghave: add a check for docker support This currently refuses to operate if on a non-Linux host. I suspect that Docker running on FreeBSD 11 or on an Illumos derivative would work fine, but I don't have ready access to such a system. On OS X using boot2docker (I used a hacky xhyve-based one for testing), it won't work because $TESTTEMP doesn't end up inside the set of directories that get forwarded to the boot2docker VM, so you can't actually drop debs in the $TESTTEMP at all. It would be possible (probably even trivial) to hack around this by using a randomly-named temporary directory inside the working directory, but that seems unlikely to be useful enough to justify the ugliness.

hghave: add a check for debian packaging tools

run-tests: add support for marking tests as very slow I want to add tests for our packaging rules, but those necessarily run a whole build, or possibly two if both native packaging and docker are available. This lets us flag such tests with a `#require slow` so that they don't unnecessarily slow down normal test runs.

builddeb: rework how output dir and platform are specified This makes it possible to write tests for both builddeb and dockerdeb that actually build .debs and then sanity check the contents.

parsers: fix two cases of unsigned long instead of Py_ssize_t We had to do this before because Python 2.4 didn't understand the n format specifier in Py_BuildValue and friends. We no longer have that problem.

templatefilters: remove redundant 'date' and 'strip' filters These filters are defined as 'date()' and 'strip()' functions. Help messages are moved to the corresponding functions.

templater: introduce unified filter syntax for unary functions "filter(expr)" is allowed already. This patch adds the opposite, "expr|func".

templater: inline getfilter() to buildfilter() This prepares for the unified filter syntax that will be introduced by the next patch.