[enriched-cocom] Propagate commit parents to enriched index

[valeriocos]

This code propogates the parents of the commit to the enriched documents. This allows to build scripts to navigate the commit graph.

Tests have been updated accordingly.

[coveralls]

Pull Request Test Coverage Report for Build 2186

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.002%) to 81.716%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/chaoss/grimoirelab-elk/grimoire_elk/enriched/cocom.py	4	92.94%
<!--	Total:	4		-->

Totals
Change from base Build 2185:	0.002%
Covered Lines:	8103
Relevant Lines:	9916

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💛 - Coveralls

[dlumbrer]

LGMT, thank you @valeriocos