Add lcov output format

[Spacetown]

This PR adds a writer for the LCOV info format as requested in #523.
The optional version line is filled with the MD5 of the file content.

Add scrubber for test suite to remove the root to the repository from output files.

Closes #523

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (737a496) 95.65% compared to head (d161f05) 95.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #830      +/-   ##
==========================================
- Coverage   95.65%   95.27%   -0.38%     
==========================================
  Files          44       54      +10     
  Lines        4347     4469     +122     
  Branches      852      873      +21     
==========================================
+ Hits         4158     4258     +100     
- Misses        111      127      +16     
- Partials       78       84       +6     

Files	Coverage Δ
gcovr/configuration.py	99.50% <ø> (ø)
gcovr/formats/cobertura/__init__.py	100.00% <ø> (ø)
gcovr/formats/coveralls/__init__.py	100.00% <ø> (ø)
gcovr/formats/gcov/__init__.py	100.00% <ø> (ø)
gcovr/formats/jacoco/__init__.py	100.00% <ø> (ø)
gcovr/formats/json/__init__.py	100.00% <ø> (ø)
gcovr/formats/lcov/__init__.py	100.00% <100.00%> (ø)
gcovr/formats/txt/__init__.py	100.00% <ø> (ø)
gcovr/tests/test_gcovr.py	98.83% <100.00%> (+0.02%)	⬆️
gcovr/formats/__init__.py	90.12% <66.66%> (-0.91%)	⬇️
... and 1 more

... and 8 files with indirect coverage changes

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[Spacetown]

@latk In the linked man page man 1 geninfo of #523 there is no VER:... but in the man page I used it's added. The VSCode plugin I doesn't understand this line. Shall I comment it since it'sto new and some tools wouldn't understand the file?

[latk]

In the linked man page man 1 geninfo of #523 there is no VER:... but in the man page I used it's added. The VSCode plugin I doesn't understand this line. Shall I comment it since it'sto new and some tools wouldn't understand the file?

The optional VER: field was added to man 1 geninfo fairly recently (this January) in this commit: linux-test-project/lcov@d98659a

I think either approach for dealing with this is OK:

• It is perfectly reasonable to say that this field is optional, and for maximum compatibility with strict lcov-info consumers gcovr won't emit it.
• It is also perfectly reasonable to target a specific lcov-info format version – gcovr can't support all historical versions of that format, just like gcovr doesn't support ancient GCC versions. We might as well target the newest format existing now, emit the field, and the rest of the ecosystem will eventually catch up.

Personally, I'd tend towards the latter, for two reasons:

• Lcov itself will emit this field, so we should as well (if this can be done in a compatible manner).
• Tools that consume the lcov-info format should skip unrecognized lines, taking into account Postel's law. Similarly, gcovr can skip over unrecognized gcov lines.

[Spacetown]

If there are problems with the field we can add an option to skip it.

[Spacetown]

@latk I've added an upload of a reference file to Codecov.