NormalMerge should call callback on conflicts

Reproduction steps

        [Fact]
        public void NormalMergeShouldCallCallbackOnConflicts()
        {
            string committishToMerge = "fast_forward";

            using (var repo = new Repository(SandboxMergeTestRepo()))
            {
                Touch(repo.Info.WorkingDirectory, "b.txt", "this is an alternate change");
                var conflicts = 0;
                var options = new MergeOptions
                {
                    OnCheckoutNotify = (path, flags) =>
                {
                    conflicts++;
                    return true;
                },
                CheckoutNotifyFlags = CheckoutNotifyFlags.Conflict,
                FastForwardStrategy = FastForwardStrategy.NoFastForward,
            };

                Assert.Throws<CheckoutConflictException>(() => repo.Merge(committishToMerge, Constants.Signature, options));
                Assert.Equal(1, conflicts) ;
            }
        }

Expected behavior

Test shouldn't fail

Actual behavior

Test fails

Version of LibGit2Sharp (release number or SHA1)

df3b22a754ef56da8d7e3c330ce2d783c2b7982e

Operating system(s) tested; .NET runtime tested

Windows netcore 2.1

Beginne mit dem Test NormalMergeShouldCallCallbackOnConflicts und dem Aufruf von repo.Merge mit MergeOptions.OnCheckoutNotify, CheckoutNotifyFlags.Conflict und NoFastForward. Reproduziere den Fehler, verfolge die Konfliktbehandlung und bestätige, dass der Callback einmal ausgeführt wird, während CheckoutConflictException weiterhin ausgelöst wird.