Use yaml anchors, aliases, and merge to avoid repetition.

RELNOTES: None.
PiperOrigin-RevId: 252430973

.bazelci/presubmit.yml

@@ -1,34 +1,32 @@
 ---
-tasks:
-  macos_latest:
-    name: "Latest Bazel"
+x_defaults:
+  # YAML has a feature for "repeated nodes", BazelCI is fine with extra nodes
+  # it doesn't know about; so that is used to avoid repeating common subparts.
+  common: &common
     platform: macos
-    bazel: latest
     build_targets:
     - "//..."
     test_targets:
     - "//..."
 
+tasks:
+  macos_latest:
+    name: "Latest Bazel"
+    bazel: latest
+    <<: *common
+
   macos_last_green:
     name: "Last Green Bazel"
-    platform: macos
     bazel: last_green
-    build_targets:
-    - "//..."
-    test_targets:
-    - "//..."
+    <<: *common
 
   macos_latest_head_deps:
     name: "Latest Bazel with Head Deps"
-    platform: macos
     bazel: latest
     shell_commands:
     # Update the WORKSPACE to use head versions of some deps to ensure nothing
     # has landed on them breaking this project.
     - .bazelci/update_workspace_to_deps_heads.sh
-    build_targets:
-    - "//..."
-    test_targets:
-    - "//..."
+    <<: *common
 
 buildifier: latest