Merge branch 'stable-2.15' into stable-2.16

* stable-2.15:
  Upgrade JGit to 4.11.9.201909030838-r
  Upgrade JGit to 4.7.9.201904161809-r
  Upgrade testcontainers to 1.12.2
  Align testcontainers version constant name prefix with core's
  Upgrade bazlets to latest stable-2.15 to build with 2.15.17 API

Change-Id: I6be0c6f6a05538a23c782adf9675566eac1d6e79