tests/test-hgk.t

doc: add the tool to check section marks in help documents

This patch adds "doc/check-seclevel.py" which checks below in help
documents:

  - whether unknown or unavailable section marks are used or not
  - whether appropriate section mark is used at sub-sectioning

It should be invoked in "doc" directory.

It checks all help documents of Mercurial (topics, commands,
extensions), if no file is specified by --file option.

With --file option, it checks contents of the specified file as help
document, for self testing purpose: -t/-c/-e/-C are used to specify
what kind of help document contents of the specified file is.

This checking is related to changeset 979b107eaea2.

Minimal hgk check

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "hgk=" >> $HGRCPATH
  $ hg init repo
  $ cd repo
  $ echo a > a
  $ hg ci -Am adda
  adding a
  $ hg debug-cat-file commit 0
  tree a0c8bcbbb45c
  parent 000000000000
  author test 0 0
  committer test 0 0
  revision 0
  branch default
  
  adda

  $ cd ..