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.

- [x] add force text into code path
- [x] think about a better place to add this so there isn't redundant code
- [x] write more comprehensive tests for attempting diffs on binary data