[PATCH 0 of 1] doc: add the tool to check section marks in help documents

[FUJIWARA Katsunori]

This patch series adds "doc/check-seclevel.py" which checks section
mark usages in Mercurial help documents.

In this series, to avoid adding new test script under tests, I added
"doc/check-seclevel.py" invocation to "tests/test-help.t", because I
couldn't find other appropriate tests for it.

But "tests/test-help.t" seems to be the test only for the help
function itself.

Should I add it to another one or add new test script under tests ?