revlog: add a C implementation of `headrevsdiff` Python implementation of `headrevsdiff` can be very slow in the worst case compared with the `heads` computation it replaces, since the latter is done in C. Even the average case of this Python implementation is still noticeable in the profiles. This patch makes the computation much much faster by doing it in C.

unbundle: faster computation of changed heads To compute the set of changed heads it's sufficient to look at the recent commits, instead of looking at all heads currently in existence.

branching: merge stable into default

hg-core: separate timestamp and extra methods

debugformat: fix formatting for compression level `bytes(<int>)` gives a very different result as `str(<int>)` and the display of `hg debugformat` have been broken for a while as a result.

hg-core: implement timestamp line parsing

doc: document that labels must have a dot in them to have an effect I noticed that the `hg topics` template has a bare `topic` label with no dot, and that makes it useless, as such a label will never receive any effect by the colour extension. This dot has been required for a long time, at least since 2011, but we never formally documented it!

tests: tweak chg test to make it fail less often the test apparently sometimes prints the word "start" as a part of profile, so let's no longer match "start": CHGHG=/*/install/bin/hg (glob) + \x1b[90m | 50.0% 0.01s profiling.py: __enter__ line 196: self.start()\x1b[0m (esc) + \x1b[90m | 50.0% 0.01s profiling.py: start line 261: self._profiler.__enter__()\x1b[0m (esc) + \x1b[90m | 50.0% 0.01s profiling.py: statprofile line 125: statprof.start(mechanism=b'...\x1b[0m (esc) + \x1b[90m | 50.0% 0.01s statprof.py: start line 356: state.thread.start()\x1b[0m (esc) + \x1b[90m | 50.0% 0.01s threading.py: start line 852: self._started.wait()\x1b[0m (esc)