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

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

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<?xml version='1.0' encoding='utf-8'?>

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<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

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  <variables/>

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  <globals>

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    <xhtml:p><![CDATA[

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/*

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    Web interface is build as SVG file in Inkscape. 

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    To edit SVG file click 'Inkscape' button on the toolbar above.

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    Inkscape is a free and open-source vector graphics editor. 

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    It's not part of Beremiz and needs to be installed separately.

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*/

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]]></xhtml:p>

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  </globals>

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  <init>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </init>

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  <cleanup>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </cleanup>

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  <start>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </start>

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  <stop>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </stop>

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</PyFile>