This refactors the diff output code so that an iterator object
can be used to traverse and generate the diffs, instead of just
the `foreach()` style with callbacks. The code has been rearranged
so that the two styles can still share most functions.
This also replaces `GIT_REVWALKOVER` with `GIT_ITEROVER` and uses
that as a common error code for marking the end of iteration when
using a iterator style of object.
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