Fixes #2452 - Skipping SHA256 tests if unsupported

The test suite was broken e.g. on Debian 12 due to requiring a very
recent version of Git installed on the system. This commit skips SHA256
tests in the git module, if a Git version older than 2.42 or gogit is used.

modules/git/utils_test.go

@@ -0,0 +1,16 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import "testing"
+
+// This file contains utility functions that are used across multiple tests,
+// but not in production code.
+
+func skipIfSHA256NotSupported(t *testing.T) {
+	if isGogit || CheckGitVersionAtLeast("2.42") != nil {
+		t.Skip("skipping because installed Git version doesn't support SHA256")
+	}
+}
+