crlf: pass-through mixed EOL buffers from LF->CRLF

Files with mixed line endings are neither CRLF nor LF files, so the only
sensible thing we can do is refuse to change the contents.

This commit changes the LF->CRLF filter.

---

This one is the one which would cause the most issues. We would ideally also make the LF->CRLF filter bail on mixed line endings, but from my reading of `src/crlf.c` in that case we bail in `apply_crlf_to_odb` before getting to the filter. We may still apply the filter in `git_blob_create_fromchunks`.

We don't seem to have tests for mixed files on checkout which run on a unix platform, so this may cause those tests to fail on Windows.

From #2393