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.