feat: avoid pushing release branch only for rebasing (#114)

Right now releaser-pleaser pushes the branch even if it is only for a "rebase",
this wastes CI resources. Instead, it should only push when there are changes
to the files it owns.

- **Old**: Push when there is a diff origin/release-pr..release-pr
- **New**: Push when the these two diffs are not the same:
  
      origin/main..release-pr
      $(git merge-base origin/main origin/release-pr)..release-pr

Closes #92

releaserpleaser.go

@@ -269,7 +269,7 @@ func (rp *ReleaserPleaser) runReconcileReleasePR(ctx context.Context) error {
 	logger.InfoContext(ctx, "created release commit", "commit.hash", releaseCommit.Hash, "commit.message", releaseCommit.Message, "commit.author", releaseCommitAuthor)
 
 	// Check if anything changed in comparison to the remote branch (if exists)
-	newReleasePRChanges, err := repo.HasChangesWithRemote(ctx, rpBranch)
+	newReleasePRChanges, err := repo.HasChangesWithRemote(ctx, rp.targetBranch, rpBranch)
 	if err != nil {
 		return err
 	}