topics: add a config knob to forbid untopiced commit
After this patch one can add the following to their hgrc to stop allowing new
commits with no topic on it.
[experimental]
enforce-topic = yes
all: tutorials/tutorial.rst static/logo-evolve.ico
sphinx-build . ../html/
tutorials/tutorial.rst: tutorials/tutorial.t
python test2rst.py tutorials/
static/logo-evolve.ico: static/logo-evolve.svg
convert -resize 36x36 static/logo-evolve.svg static/logo-evolve.ico