Bug 1731178 - [taskgraph] Explicitly set base revision in tgdiff task, r=taskgraph-reviewers,bhearsum

Since all commits on non-try branches are public, vcs.base_ref was returning
the current revision each time and the diff task was always diffing against the
same revision twice.

By using 'json-automationrelevance' to determine the parent push, we can be
sure we're always diffing against the proper revision.

Depends on D125974

Differential Revision: https://phabricator.services.mozilla.com/D125975

taskcluster/ci/source-test/taskgraph.yml

@@ -27,7 +27,7 @@ diff:
         cwd: '{checkout}'
         command: >-
             mkdir -p /builds/worker/artifacts/diffs &&
-            ./mach taskgraph target --fast -p "taskcluster/test/params" -o "/builds/worker/artifacts/diffs/diff_{params}.txt" --diff
+            ./mach taskgraph target --fast -p "taskcluster/test/params" -o "/builds/worker/artifacts/diffs/diff.txt" --diff {base_rev}
     when:
         files-changed:
             - 'taskcluster/ci/**'

taskcluster/taskgraph/transforms/source_test.py

@@ -10,9 +10,11 @@ treeherder configuration and attributes for that platform.
 import copy
 import os
 
+import taskgraph
 from taskgraph.transforms.base import TransformSequence
 from taskgraph.transforms.job import job_description_schema
 from taskgraph.util.attributes import keymatch
+from taskgraph.util.hg import get_json_automationrelevance
 from taskgraph.util.schema import (
     resolve_keyed_by,
     optionally_keyed_by,
@@ -243,3 +245,25 @@ def set_code_review_env(config, jobs):
             env["CODE_REVIEW"] = "1"
 
         yield job
+
+
+@transforms.add
+def set_base_revision_in_tgdiff(config, jobs):
+    # Don't attempt to download 'json-automation' locally as the revision may
+    # not exist in the repository.
+    if not os.environ.get("MOZ_AUTOMATION") or taskgraph.fast:
+        yield from jobs
+        return
+
+    data = get_json_automationrelevance(
+        config.params["head_repository"], config.params["head_rev"]
+    )
+    for job in jobs:
+        if job["name"] != "taskgraph-diff":
+            yield job
+            continue
+
+        job["run"]["command-context"] = {
+            "base_rev": data["changesets"][0]["parents"][0]
+        }
+        yield job