contrib/docker/debian.template

filemerge: move decorator definition for internal merge tools to registrar

This patch also adds extra loading entry for internal merge tools to
extensions.py, for similarity to other decorators defined in
registrar.py.

This patch uses "internalmerge" for decorator class name, instead of
original "internaltool", because the latter is too generic.

BTW, after this patch, 4-spaces indentation is added to the 1st line
of internal merge tool description docstring, and this may make
already translated entries in *.po fuzzy.

Even though this indentation is required for "definition list" in reST
syntax, absence of it has been overlooked, because help.makeitemsdoc()
forcibly inserts it at generation of online help.

But this forcible insertion causes formatting issue (I'll send another
patch series for this). Therefore, this additional indentation should
be reasonable.

FROM debian:__CODENAME__
RUN apt-get update && apt-get install -y \
  build-essential \
  debhelper \
  dh-python \
  devscripts \
  less \
  python \
  python-all-dev \
  python-docutils \
  zip \
  unzip