[Datumaro] Fix merge command cli

[zhiltsov-max]

Motivation and context

Fixes #2020 (revert from #1973)

How has this been tested?

Manual test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6890

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 68.906%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/cli/contexts/project/init.py	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	7	75.52%

Totals
Change from base Build 6871:	-0.03%
Covered Lines:	11122
Relevant Lines:	15658

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💛 - Coveralls