ci/eval: name the base branch in the performance comparison summary

The performance comparison summary previously stated it compared against
"its pull request base branch (e.g., 'master')" regardless of the actual
base branch.

Plumb the base branch classification computed in prepare.js through the
eval workflow to the `compare` derivation so the summary names the real
base branch (e.g. "staging-25.11"). The local `eval.full` helper defaults
to "master", matching its touched-files convention.

Assisted-by: claude-code with claude-opus-4-8[1m]-high
(cherry picked from commit a100d091a0)

.github/workflows/eval.yml

@@ -15,6 +15,9 @@ on:
       targetSha:
         required: true
         type: string
+      baseBranch:
+        required: true
+        type: string
       systems:
         required: true
         type: string
@@ -291,6 +294,7 @@ jobs:
       - name: Compare against the target branch
         env:
           TARGET_SHA: ${{ inputs.mergedSha }}
+          BASE_BRANCH: ${{ fromJSON(inputs.baseBranch).branch }}
         run: |
           git -C nixpkgs/trusted diff --name-only "$TARGET_SHA" \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
@@ -299,6 +303,7 @@ jobs:
           nix-build nixpkgs/trusted/ci --arg nixpkgs ./nixpkgs/trusted-pinned -A eval.compare \
             --arg combinedDir ./combined \
             --arg touchedFilesJson ./touched-files.json \
+            --argstr baseBranch "$BASE_BRANCH" \
             --out-link comparison
 
           cat comparison/step-summary.md >> "$GITHUB_STEP_SUMMARY"

.github/workflows/merge-group.yml

@@ -97,6 +97,7 @@ jobs:
       artifact-prefix: ${{ inputs.artifact-prefix }}
       mergedSha: ${{ needs.prepare.outputs.mergedSha }}
       targetSha: ${{ needs.prepare.outputs.targetSha }}
+      baseBranch: ${{ needs.prepare.outputs.baseBranch }}
       systems: ${{ needs.prepare.outputs.systems }}
 
   build:

.github/workflows/pull-request-target.yml

@@ -110,6 +110,7 @@ jobs:
       mergedSha: ${{ needs.prepare.outputs.mergedSha }}
       headSha: ${{ github.event.pull_request.head.sha }}
       targetSha: ${{ needs.prepare.outputs.targetSha }}
+      baseBranch: ${{ needs.prepare.outputs.baseBranch }}
       systems: ${{ needs.prepare.outputs.systems }}
       testVersions: ${{ contains(fromJSON(needs.prepare.outputs.touched), 'pinned') && !contains(fromJSON(needs.prepare.outputs.headBranch).type, 'development') }}
 

ci/eval/compare/default.nix

@@ -49,6 +49,7 @@ in
 {
   combinedDir,
   touchedFilesJson,
+  baseBranch,
   ownersFile ? ../../OWNERS,
 }:
 let
@@ -240,7 +241,7 @@ runCommand "compare"
       echo
       echo "# Performance comparison"
       echo
-      echo "This compares the performance of this branch against its pull request base branch (e.g., 'master')"
+      echo "This compares the performance of this branch against the \`${baseBranch}\` branch."
       echo
     } >> $out/step-summary.md
 

ci/eval/default.nix

@@ -288,6 +288,9 @@ let
       #   | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
       # ```
       touchedFilesJson ? builtins.toFile "touched-files.json" "[ ]",
+      # The branch the local comparison is made against; matches the `master`
+      # used in the touched-files expression above.
+      baseBranch ? "master",
     }:
     let
       diffs = symlinkJoin {
@@ -305,7 +308,7 @@ let
       };
       comparisonReport = compare {
         combinedDir = combine { diffDir = diffs; };
-        inherit touchedFilesJson;
+        inherit touchedFilesJson baseBranch;
       };
     in
     comparisonReport;