evolve: remove the unnecessary condition to check empty successors set
I annotated the history of this "newer == [()]" condition and found
that it was added in the initial stages of evolution project (more than
8 years ago) and there is no test where we get [()] as a successor set.
So looks like "if not newer" is enough to check if it is empty.
I also looked into obsutil.successorssets() implementation and
I don't think it would give us [()] in any case.
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.