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

Start with the NormalMergeShouldCallCallbackOnConflicts test and the repo.Merge call using MergeOptions.OnCheckoutNotify, CheckoutNotifyFlags.Conflict, and NoFastForward. Reproduce the failure, trace conflict handling, and confirm the callback runs once while CheckoutConflictException is still raised.