should not open filehandles against character-containing strings without declaring the encoding

I got these errors during a release:
    Strings with code points over 0xFF may not be mapped into in-memory file handles
    Can't open 'SCALAR(0x105d2e518)' for reading: 'Invalid argument' at /Users/ether/.perlbrew/libs/19.3@std/lib/perl5/Dist/Zilla/Plugin/Git/CheckFor/MergeConflicts.pm line 68

...because my files contained unicode characters that were beyond the latin1
range (yay Russian canaries).

This change splits the file into lines using split, rather than opening a
filehandle to its reference.

It also skips over any files marked as encoding=bytes (Dist::Zilla 5.000+),
whose content is not introspectable.