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

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

include yslt.yml2

in xsl {
    decl istylesheet (
        *output="xml",
        version="1.0",
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform",
        xmlns:exsl='http://exslt.org/common',
        extension-element-prefixes='exsl'
    ) alias stylesheet {
        output *output;
        content;
    };

    decl template(match) {
        content;
    };

    decl function(name) alias template {
        content;
    };
   
    decl call(name) alias call-template {
        content;
    };

    decl apply(select) alias apply-templates {
        content;
    };
}