Expose commit parents attribute in commit table

[mcarmonaa]

A new attribute parents has been added to the commits table containing an array with each commit's parent hashes.

MySQL [(none)]> DESCRIBE TABLE commits;
+------+------+
| name | type |
+------+------+
| hash | TEXT |
| author_name | TEXT |
| author_email | TEXT |
| author_when | TIMESTAMP |
| committer_name | TEXT |
| committer_email | TEXT |
| committer_when | TIMESTAMP |
| message | TEXT |
| tree_hash | TEXT |
| parents | JSON |
+------+------+
10 rows in set (0.00 sec)

MySQL [(none)]> select hash,committer_name,parents from commits where array_length(parents) > 1 limit 10;
+------+----------------+---------+
| hash | committer_name | parents |
+------+----------------+---------+
| c1a24566f4a89001ade1441dfbd7d98a2bd93ba8 | R.J.V. Bertin  | ["e997744cc641628d28928d830ecf7ec619308d57","6c92e0f8307e79564fe305dee6b3ad5f3df11fb7"] |
| fc56503d7278925acac8f92749f0808db7ffa248 | René J.V. Bertin | ["dc876b06c1cca76d0fcab99588cff3afcdb2f39b","67809e220b47bae81e726d170fd5beb1b41dbeb5"] |
| 81b0c971b414f2d3687c9c837a3b150cfbe4c9c8 | Alexander Potashev | ["b6913c136d8d2ca1216b8c64790b2832f337931a","c8a7b1e5dca1ba6a04c2f117e7d09c607ba15f32"] |
| f1fa0c18ab2148d208920aeaa8787548ed47ae88 | Michael Palimaka | ["9a190c14d853b46c24d326063f913c3efe384f9b","2dce7ebe83e24451053aaa5ffe3822b5f1cb14b9"] |
| 8a4e4ab5e2788fbd688a2d161bb2f70ab2c8e7f1 | Harald Sitter  | ["a69a759297b0b936617456712abf98b7efdd90a0","496f618a970c17568976b4fece4222e4ac55257b"] |
| afd851908793496f47d921c02fa56855a1200cc2 | Aleix Pol      | ["ba6956e6242d25360110a9bbf838d2eb3fb77479","73df960e45ff0bf58865c18be43381d8f4da3d8e"] |
| 7d15e6a9d071032a8783471b4e3518cf5388dfaf | Lingerhk       | ["9189de7f9cb11ebb6fc6cf304c035d5f1f41b9ed","3eae80004b064f155604519d5a92ad80a0f521f7"] |
| 7a3b4628002d59e8a6faa0bb057873c95a5c5b8d | wangbo         | ["be3476f843480c101d5291317df364460d7abec1","1008be932248a16e5a9500be4129efeeb4564b39"] |
| 01348bced4cd560ca86a02bf484988da134f7d3f | wangbo         | ["fff7062de8474d10a67d417ccea87ba6f58ca81d","563524c36b5e80acfd1ff0aca2386e934e4fa625"] |
| d25925ced790fa79290dc47de3336cdb73004061 | linky          | ["42c74a8b0b7e8d0afc3275e409bfba5c15fe8af6","cd9e145151fb50885fc17294dc57746fcce04801"] |
+------+----------------+---------+
10 rows in set (0.16 sec)

Closes #226

[mcarmonaa]

@src-d/data-retrieval please review (for some reason the "reviewers searcher" doesn't work)

[smola]

Add a test that uses it and checks its result?

[erizocosmico]

LGTM after tests for this are added, as @smola said

[mcarmonaa]

@smola @erizocosmico I already added the test 👍