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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/* Beremiz' header file for use by extensions */

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#include "iec_types.h"

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#define LOG_LEVELS 4

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#define LOG_CRITICAL 0

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#define LOG_WARNING 1

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#define LOG_INFO 2

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#define LOG_DEBUG 3

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extern unsigned long long common_ticktime__;

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#ifdef TARGET_LOGGING_DISABLE

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#define LogMessage(level, buf, size)

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#else

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int     LogMessage(uint8_t level, char* buf, uint32_t size);

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#endif

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long AtomicCompareExchange(long* atomicvar,long compared, long exchange);

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