Add command line argument to run linter only on changed files

./tests/tools/check_source.sh --only-changes

It's recommended to run check_source.sh automatically on each commit,
so the rules are always enforced.
Even better is to use docker for that, so the versions of pep8 and
pylint are the same as on pipeline server.

To do this couple of lines should be added into repository's hgrc file.

[----------- cut from .hg/hgrc------------------]

[hooks]
precommit.linter = ./tests/tools/check_source.sh --only-changes
# precommit.linter = docker run -it --volume=$PWD:/beremiz --workdir="/beremiz" --volume=$PWD/../CanFestival-3:/CanFestival-3 --memory=1g --entrypoint=/beremiz/tests/tools/check_source.sh skvorl/beremiz-requirements --only-changes

[-----------------------------------------------]

image: skvorl/beremiz-requirements

pipelines:
  custom: # Pipelines that are triggered manually
    checks: # The name that is displayed in the list in the Bitbucket Cloud GUI
        - step:
            script: # Modify the commands below to build your repository.
              - ln -s /CanFestival-3 $BITBUCKET_CLONE_DIR/../CanFestival-3
              - /usr/bin/python --version
              - ./tests/tools/check_source.sh
              - ./tests/tools/run_python_tests.sh


  default:
    - step:
        script: # Modify the commands below to build your repository.
          - ln -s /CanFestival-3 $BITBUCKET_CLONE_DIR/../CanFestival-3
          - /usr/bin/python --version
          - ./tests/tools/check_source.sh
          - ./tests/tools/run_python_tests.sh