Make core.safecrlf work on LF-ending platforms

If you enabled core.safecrlf on an LF-ending platform, we would
error even for files with all LFs.  We should only be warning on
irreversible mappings, I think.
Just don't CRLF filter if there are no CRs
Merge pull request #2375 from libgit2/rb/safecrlf-on-lf-platform

Make core.safecrlf not generate an error on LF-ending platforms