Add FORCE_TEXT check into git_diff_blobs code path

`git_diff_blobs` and `git_diff_blob_to_buffer` skip the step
where we check file attributes because they don't have a filename
associated with the data. Unfortunately, this meant they were also
skipping the check for the GIT_DIFF_FORCE_TEXT option and so you
could not force a diff of an apparent binary file.  This adds the
force text check into their code path.
Reorganize FORCE_TEXT diff flag checks
More diff tests with binary data
Merge pull request #1246 from arrbee/fix-force-text-for-diff-blobs

Add FORCE_TEXT check into git_diff_blobs code path