makefile: rework "{generate,vendor}-check" targets

`TMP_DIR := $(shell mktemp -d)` causes everytime we call `make` to leak
a temporary directory "/tmp/tmp.*/". We could fix that by always using
the same directory ("/tmp/something") or by concatenating all make steps
with `TMPDIR=$(mktemp ...) && cd $TMPDIR && ...`.

Instead, add and use a "scripts/check-gittree-for-diff.sh" script.

That is useful, because the make targets "{generate,vendor}-check" are
basically the same. We can implement them both by calling the script.
And by having a stand-alone bash scripts, it's easier to review (as it's
not a shell scripted escaped in inside make).

Also, `make vendor-check` now just calls `make vendor`. Previously, it
reimplemnted how vendoring works.

Makefile

@@ -156,24 +156,13 @@ vendor:
 generate: vendor controller-gen ## Generate code containing DeepCopy, DeepCopyInto, and DeepCopyObject method implementations.
 	GOFLAGS='' $(CONTROLLER_GEN) object:headerFile="hack/boilerplate.go.txt" paths="./..."
 
-TMP_DIR := $(shell mktemp -d)
 .PHONY: generate-check
 generate-check: controller-gen
-	rm -rf $(TMP_DIR)
-	mkdir -p $(TMP_DIR)
-	cp -r . $(TMP_DIR)
-	cd $(TMP_DIR) && make generate
-	diff -r . $(TMP_DIR) || exit 1
-	rm -rf $(TMP_DIR)
+	./scripts/check-gittree-for-diff.sh make generate
 
 .PHONY: vendor-check
 vendor-check:
-	rm -rf $(TMP_DIR)
-	mkdir -p $(TMP_DIR)
-	cp -r . $(TMP_DIR)
-	cd $(TMP_DIR) && go mod vendor && go mod tidy
-	diff -r . $(TMP_DIR) || exit 1
-	rm -rf $(TMP_DIR)
+	./scripts/check-gittree-for-diff.sh make vendor
 
 .PHONY: fmt
 fmt: ## Run go fmt against code.

scripts/check-gittree-for-diff.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/bash
+
+die() {
+    printf "%s\n" "$*"
+    exit 1
+}
+
+set -e
+
+test $# -gt 0 || die "Usage: $0 CMD..."
+
+cd "$(dirname "$0")/.."
+
+DIR="$(mktemp -t -d dpu-operator-check-gittree-for-diff.XXXX)"
+
+cp -ar ./ "$DIR/"
+
+echo "Checking \`$@\` in \"$DIR\""
+
+pushd "$DIR/" 1>/dev/null
+"$@"
+popd 1>/dev/null
+
+diff -r . "$DIR/" || die "There is a difference between \"$PWD\" and \"$DIR\" after calling \`$@\`"
+
+rm -rf "$DIR/"