Pre-emptively copy data when marshalling diff callback data.

When marshalling diff callback data to Go structs, any `char*` need to
be pre-emptively copied onto Go's heap as they're invalidated as soon as
our callback function returns. This patch adds this extra copy before
sending the value to the channel, which fixes a bug wherein
`DiffLine.Content`, `DiffFile.Path` and `DiffHunk.Header` would
previously return garbage data.