Add new GetPatchID RPC

This MR adds GetPatchID RPC that allows us to get the patch-id for a patch. It can be used to compare diffs to see if they are the same or not by comparing its patch-id.

Closes #4612 (closed)

internal/gitaly/service/diff/patch_id_test.go

@@ -5,8 +5,8 @@ package diff
 import (
 	"testing"
 
-	"gitlab.com/gitlab-org/gitaly/v15/internal/structerr"
 	"gitlab.com/gitlab-org/gitaly/v15/internal/helper"
+	"gitlab.com/gitlab-org/gitaly/v15/internal/structerr"
 	"gitlab.com/gitlab-org/gitaly/v15/internal/testhelper"
 	"gitlab.com/gitlab-org/gitaly/v15/proto/go/gitalypb"
 )