In the process of adding tests for the max file size threshold
(which treats files over a certain size as binary) there seem to
be a number of problems in the new code with detecting binaries.
This should fix those up, as well as add a test for the file
size threshold stuff.
Also, this un-deprecates `GIT_DIFF_LINE_ADD_EOFNL`, since I
finally found a legitimate situation where it would be returned.
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.
