Update git submodules
Update git submodules

* Update plugins/gitiles from branch 'master'
  to 860c74e31d41c9ff6b3b39c99b40647fd7fce9d7
  - Merge branch 'stable-3.1'
    
    * stable-3.1:
      Bump Bazel version to 2.2.0
      Bump required Bazel version to 2.1.0
      Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
      Documentation: Add missing footer that links to plugin index page
      Documentation: Refactor to match other plugin documentation
      Add an empty tools/BUILD file explicitly for Bazel
      Upgrade bazlets to latest stable-2.16
      Upgrade bazlets to latest stable-2.16
      Upgrade bazlets to latest stable-2.15
      Upgrade bazlets to latest stable-2.14
    
    Change-Id: I436825569e983e06b338030d135e99393925f829
    
  - Merge branch 'stable-3.0' into stable-3.1
    
    * stable-3.0:
      Bump Bazel version to 2.2.0
      Bump required Bazel version to 2.1.0
      Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
      Documentation: Add missing footer that links to plugin index page
      Documentation: Refactor to match other plugin documentation
      Add an empty tools/BUILD file explicitly for Bazel
      Upgrade bazlets to latest stable-2.16
      Upgrade bazlets to latest stable-2.16
      Upgrade bazlets to latest stable-2.15
      Upgrade bazlets to latest stable-2.14
    
    Change-Id: Iee7393c165edb1029e34ca4742d381731871c300
    
  - Merge branch 'stable-2.16' into stable-3.0
    
    * stable-2.16:
      Bump Bazel version to 2.2.0
      Bump required Bazel version to 2.1.0
      Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
      Documentation: Add missing footer that links to plugin index page
      Documentation: Refactor to match other plugin documentation
      Add an empty tools/BUILD file explicitly for Bazel
      Upgrade bazlets to latest stable-2.16
      Upgrade bazlets to latest stable-2.16
      Upgrade bazlets to latest stable-2.15
      Upgrade bazlets to latest stable-2.14
    
    Change-Id: Ia85129611b6f4b110e2b298c108d23668b1908a5
    
  - Bump Bazel version to 2.2.0
    
    Change-Id: I409883c778236176cb80368e71c2df0b877b395f
    
  - Bump required Bazel version to 2.1.0
    
    Change-Id: I2b85f666c39fc7d6d69022dcd695e07891fe9352
    
  - Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
    
    Change-Id: I204aa0b0784c953ae52f7db62907dda81690af22
    
  - Documentation: Add missing footer that links to plugin index page
    
    Change-Id: If5be574b2848ba308227e2019fcf2162693e36cc
    
  - Documentation: Refactor to match other plugin documentation
    
    Replace hard-coded command for plugin in-tree build.
    
    Change-Id: Icd2027015c55d5adfe4bbb2950284d4f8743215e
    
  - Add an empty tools/BUILD file explicitly for Bazel
    
    To make sure that all plugins consistently have empty BUILD files
    within their tools.
    
    Change-Id: I50809c0235f8de7f86e9ee4bffd32397b393afa3
    
  - Upgrade bazlets to latest stable-2.16
    
    As the previously used commit was locally verified but not the one
    merged in after reviewing.
    
    Change-Id: Ie5318222100ee61efa3b4edda02d66396bc818a8
    
  - Upgrade bazlets to latest stable-2.16
    
    To use the now mandatory https protocol for MAVEN_CENTRAL.
    
    Bug: Issue 12170
    Change-Id: I4d587f6cb68475408eaddc47be0ccbe48e23b82d
    
  - Merge branch 'stable-2.15' into stable-2.16
    
    * stable-2.15:
      Upgrade bazlets to latest stable-2.15
      Upgrade bazlets to latest stable-2.14
    
    Change-Id: Ia796ff684b366d14a5c83c4ffd685990a27b7048
    
  - Upgrade bazlets to latest stable-2.15
    
    To use the now mandatory https protocol for MAVEN_CENTRAL.
    
    Bug: Issue 12170
    Change-Id: Ib095fee3b86ec63b43bd871845efc6058e1e2c65
    
  - Merge branch 'stable-2.14' into stable-2.15
    
    * stable-2.14:
      Upgrade bazlets to latest stable-2.14
    
    Change-Id: I38a326f8bea6da058a37e70eedc54ad3a7148e00
    
  - Upgrade bazlets to latest stable-2.14
    
    To use the now mandatory https protocol for MAVEN_CENTRAL.
    
    Bug: Issue 12170
    Change-Id: I1ee528cb199eb1bf5153a60f1e1adfe09ee1d6a7
    
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