Merge branch 'stable-2.15' into stable-2.16
* stable-2.15:
ElasticContainer: Upgrade to 6.8.2 image for V6_8 tests
Fix typo: program
Clarify usage of 'parent' option in list files API
Remove unused Skylark patch file
Files: Use Gerrit API to get revision parents
Add support for Elasticsearch version 6.8.x
Upgrade elasticsearch-rest-client to 7.2.1
CommitApi: Add method to get commit info
Consolidate all CommitApi tests into a single class
Files: Validate parent option to prevent internal server error
RevisionIT: Assert that files(base) only works for patch set revisions
Fix and expand documentation of REST API to get revision files
RevisionIT#files: Simplify assertion
Update git submodules
Also update ElasticV6QueryProjectsTest to use V6_8.
Change-Id: I98da65486f39acad509529ff90540d9f8ae00a2d
