Calculate the previous commit to diff against

The github.event.pull_request.base.sha provides a wrong commit SHA
pointing at the *current* upstream HEAD.  This particular one may not be
even fetched (a concurrent PR merged in the meantime).

The GitHub Context provides no info about the previous commit SHA we
need, so let's calculate it manually from
the ${{ github.event.commits }} number

action.yml

@@ -55,10 +55,10 @@ inputs:
     default: ${{ github.event.pull_request.head.sha }}
     required: false
 
-  pull_request_base:
-    description: "The commit right before the pull-request commits (intenral)"
-    default: ${{ github.event.pull_request.base.sha }}
-    required: true
+  pull_request_commits:
+    description: "Number of commits in this PR"
+    required: false
+    default: ${{ github.event.commits }}
 
 outputs:
   sarif:

container/cmd

@@ -42,7 +42,6 @@ set_linter_options()
     for tag in $INPUT_LINTER_TAGS; do
         linter_options+=( --linter-tag "$tag" )
     done
-    linter_options+=( --compare-against "$INPUT_PULL_REQUEST_BASE" )
 }
 
 populate_sarif_data()
@@ -89,6 +88,10 @@ if test "$GITHUB_EVENT_NAME" = 'pull_request'; then
     # We don't want to rebase on top of the current upstream's head; the line
     # numbers would be shifted and the provided output would be misleading.
 
+    # Get the last commit before this PR.
+    last_committed=$(git rev-parse "HEAD~$INPUT_PULL_REQUEST_COMMITS")
+    linter_options+=( --compare-against "$last_committed" )
+
     test -z "$INPUT_SUBDIRECTORY" || INPUT_SUBDIRECTORIES=$INPUT_SUBDIRECTORY
     test -z "$INPUT_SUBDIRECTORIES" && INPUT_SUBDIRECTORIES=.