exthelper: simplify the ability to register revsets
I think this is what Yuya and Boris agreed on.[1] This happens *after* the
extsetup phase now (and after the _aftercallback notifications). But this is
trivial, mergeable between exthelper instances, and doesn't need to have the
extension name supplied when registering.
[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-December/125888.html
Doc is generated with sphinx.
You can generate the doc with:
```
make
```
# Tutorials
For updating the tutorials, you need to have the docgraph extension installed
(https://pypi.python.org/pypi/hg-docgraph).
Then run the test-topic-tutorial.t and test-tutorial.t to update the output if
needed.
You'll need the dot binary (likely installed by the graphviz package in your
package manager) in order to have graphviz graphs rendered in the html output.
Simply run make in the docs directory should takes care of the conversion of
the tutorial .t files into .rst files. Then sphinx should do the rest of the
jobs by rendering graphviz graphs.