There are three actual changes in this commit:

1. When the trailing newline of a file is removed in a diff, the
   change will now be reported with `GIT_DIFF_LINE_DEL_EOFNL` passed
   to the callback.  Previously, the `ADD_EOFNL` constant was given
   which was just an error in my understanding of when the various
   circumstances arose.  `GIT_DIFF_LINE_ADD_EOFNL` is deprecated and
   should never be generated.  A new newline is simply an `ADD`.
2. Rewrote the `diff_delta__merge_like_cgit` function that contains
   the core logic of the `git_diff_merge` implementation.  The new
   version doesn't actually have significantly different behavior,
   but the logic should be much more obvious, I think.
3. Fixed a bug in `git_diff_merge` where it freed a string pool
   while some of the string data was still in use.  This led to
   `git_diff_print_patch` accessing memory that had been freed.

The rest of this commit contains improved documentation in `diff.h`
to make the behavior and the equivalencies with core git clearer,
and a bunch of new tests to cover the various cases, oh and a minor
simplification of `examples/diff.c`.