stack: fix phasecache._phasesets check logic
When _phasesets is not None, it's a list, and it contains set()s of revisions
in a specific phase, starting from public, draft, secret and so on. But since
repos are supposed to have the majority of revisions in public phase, the first
element of this list is not a (potentially huge) set, but None.
Previously this code tried to check if there's any element that is None, and
was always finding None at index 0, so the short path was executed every time
and the rest of the function was never used.
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.