Add more linters to improve code readability (#19989)

Add nakedret, unconvert, wastedassign, stylecheck and nolintlint linters to improve code readability

- nakedret - https://github.com/alexkohler/nakedret - nakedret is a Go static analysis tool to find naked returns in functions greater than a specified function length.
- unconvert - https://github.com/mdempsky/unconvert - Remove unnecessary type conversions
- wastedassign - https://github.com/sanposhiho/wastedassign -  wastedassign finds wasted assignment statements.
- notlintlint -  Reports ill-formed or insufficient nolint directives
- stylecheck - https://staticcheck.io/docs/checks/#ST - keep style consistent
  - excluded: [ST1003 - Poorly chosen identifier](https://staticcheck.io/docs/checks/#ST1003) and [ST1005 - Incorrectly formatted error string](https://staticcheck.io/docs/checks/#ST1005)

services/pull/merge.go

@@ -358,7 +358,7 @@ func rawMerge(ctx context.Context, pr *issues_model.PullRequest, doer *user_mode
 	committer := sig
 
 	// Determine if we should sign
-	signArg := ""
+	var signArg string
 	sign, keyID, signer, _ := asymkey_service.SignMerge(ctx, pr, doer, tmpBasePath, "HEAD", trackingBranch)
 	if sign {
 		signArg = "-S" + keyID
@@ -858,7 +858,7 @@ func MergedManually(pr *issues_model.PullRequest, doer *user_model.User, baseGit
 		pr.Merger = doer
 		pr.MergerID = doer.ID
 
-		merged := false
+		var merged bool
 		if merged, err = pr.SetMerged(ctx); err != nil {
 			return err
 		} else if !merged {