setup.py, make: avoid problems with outdated, existing hgext/__index__.py*
"make clean" already removed __index__.py[cdo], but not the __index__.py
(automatically generated by "python setup.py build_hgextindex").
"setup.py build_hgextindex" did not generate a new index if file
__index__.py[cdo] already existed, because if __index__.py was removed,
the compiled file containing the old information was imported and used.
Generate an empty file (with a new timestamp to generate a new .py[cdo])
instead and make mercurial.extensions ignore the unset docs attribute.
One of the problems was a failed test-help.t, to reproduce:
$ rm hgext/__index__.py*
$ echo 'docs = {"mq": "dummy"}' > hgext/__index__.py
$ make test-help.t
With this a "make clean" or "python setup.py build_hgextindex" helps.