To answer if a single given file should be ignored, the path to
that file has to be processed progressively checking that there
are no intermediate ignored directories in getting to the file
in question. This enables that, fixing the broken old behavior,
and adds tests to exercise various ignore situations.
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.
