Allow specifying a tag that should be used as a baseline for API breakage checks

Author: ahoppen

.github/workflows/soundness.yml

@@ -11,6 +11,10 @@ on:
         type: string
         description: "Path to a file that will be passed as --breakage-allowlist-path to swift package diagnose-api-breaking-changes"
         default: ""
+      api_breakage_check_baseline:
+        type: string
+        description: "The tag against which API breakages that should be used as the baseline for the API breakage check. By default the PR base is used."
+        default: ""
       api_breakage_check_container_image:
         type: string
         description: "Container image for the API breakage check job. Defaults to latest Swift Ubuntu image."
@@ -103,11 +107,16 @@ jobs:
         run: ${{ inputs.linux_pre_build_command }}
       - name: Run API breakage check
         run: |
-          git fetch ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY} ${GITHUB_BASE_REF}:pull-base-ref
+          if [[ -z '${{ inputs.api_breakage_check_baseline }}' ]]; then
+            git fetch ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY} ${GITHUB_BASE_REF}:pull-base-ref
+            BASELINE_REF='pull-base-ref'
+          else
+            BASELINE_REF='${{ inputs.api_breakage_check_baseline }}'
+          fi
           if [[ -z '${{ inputs.api_breakage_check_allowlist_path }}' ]]; then
-            swift package diagnose-api-breaking-changes pull-base-ref
+            swift package diagnose-api-breaking-changes "$BASELINE_REF"
           else
-            swift package diagnose-api-breaking-changes pull-base-ref --breakage-allowlist-path '${{ inputs.api_breakage_check_allowlist_path }}'
+            swift package diagnose-api-breaking-changes "$BASELINE_REF" --breakage-allowlist-path '${{ inputs.api_breakage_check_allowlist_path }}'
           fi
 
   docs-check: