I'm okay with only single chars, but if this is configurable in libgit2sharp then we should fail if the marshalling gives us a multibyte UTF8 char.  But I agree that this is only minimally exciting.

But we should not default to `#` in libgit2sharp.  We should default to *none*, I think, if it's not configurable.  I don't want to lose half of my commit message in Visual Studio because it wrapped and I typed a `#` somewhere.  That would be disappointing.