use-after-free bug in diff.c

I'm seeing a use-after-free bug when accessing a Patch object after its Diff had been garbage-collected.  Valgrind confirms.

The quick fix is to `strdup()` `old_file.path` and `new_file.path` (and `free` in `Patch_dealloc`).   Better would be to skip the mallocs and piggyback the `git_patch*` onto the `Patch` object and move `git_patch_free()` to `Patch_dealloc`.  Thoughts?
