[git] Skip uncloned repositories for git-branches study

[zhquan]

This code skips uncloned repositories and continues with the next repository.

Test added accordingly.

Signed-off-by: Quan Zhou [email protected]

[coveralls]

Pull Request Test Coverage Report for Build 2993739776

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 22 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.007%) to 82.385%

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Files with Coverage Reduction	New Missed Lines	%
/home/runner/work/grimoirelab-elk/grimoirelab-elk/grimoire_elk/enriched/git.py	22	72.34%
<!--	Total:	22

Totals
Change from base Build 2910390636:	0.007%
Covered Lines:	8844
Relevant Lines:	10735

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💛 - Coveralls

[jjmerchante]

It works as expected, could you update the tests?

[jjmerchante]

LGTM