Skip the SHA256 test if git has not support for it

If you are building and running the tests in an environment with an older
version of git, it might not have SHA256 support. This should not cause the
git-sizer test suite to fail as it's not an issue with git-sizer.

Detect this situation and skip the test.

Author: carlosmn

git_sizer_test.go

@@ -867,7 +867,11 @@ func TestSHA256(t *testing.T) {
 	// exist yet:
 	cmd := exec.Command("git", "init", "--object-format", "sha256", testRepo.Path)
 	cmd.Env = testutils.CleanGitEnv()
-	err = cmd.Run()
+	output, err := cmd.CombinedOutput()
+
+	if err != nil && strings.HasPrefix(string(output), "error: unknown option `object-format'") {
+		t.Skip("skipping due to lack of SHA256 support")
+	}
 	require.NoError(t, err)
 
 	timestamp := time.Unix(1112911993, 0)