Rebase the checkout on top of target branch

This change will fail the action in case of rebase conflicts, but it
seems acceptable (the PR can not be merged anyway).  If needed, we may
end-up some implementing some fallback in the future.

action.yml

@@ -47,3 +47,6 @@ outputs:
 runs:
   using: docker
   image: docker://quay.io/copr/vcs-diff-lint:latest
+  env:
+    INPUT_PULL_REQUEST_ID: ${{ github.event.pull_request.number }}
+    INPUT_PULL_REQUEST_HEAD: ${{ github.event.pull_request.head.sha }}

container/cmd

@@ -24,7 +24,7 @@ is_true() {
 }
 
 if is_true "$INPUT_DEBUG" || is_true "$RUNNER_DEBUG"; then
-    ( echo "== GitHub ENV VARS ==" ; env | grep ^GITHUB ) >&2
+    ( echo "== GitHub ENV VARS ==" ; env | grep -e ^GITHUB -e ^INPUT_ ) >&2
     set -x
 fi
 
@@ -76,7 +76,14 @@ if test "$GITHUB_EVENT_NAME" = 'pull_request'; then
     git config --global advice.detachedHead false
     git config --global init.defaultBranch main
 
-    git fetch origin "$target_branch:$target_branch" &>/dev/null
+    (
+        echo "## Rebasing $INPUT_PULL_REQUEST_HEAD onto $target_branch ##"
+        is_true "$INPUT_DEBUG" || exec &>/dev/null
+        git fetch origin "$target_branch:$target_branch"
+        git fetch origin "+$INPUT_PULL_REQUEST_HEAD:refs/remotes/pull-requests/pr-detached-$INPUT_PULL_REQUEST_ID"
+        git checkout "$INPUT_PULL_REQUEST_HEAD"
+        git rebase "origin/$target_branch"
+    )
 
     test -z "$INPUT_SUBDIRECTORY" || INPUT_SUBDIRECTORIES=$INPUT_SUBDIRECTORY
     test -z "$INPUT_SUBDIRECTORIES" && INPUT_SUBDIRECTORIES=.