[lit] Clean up internal diff's encoding handling

As suggested by rnk at D67643#1673043, instead of reading files
multiple times until an appropriate encoding is found, read them once
as binary, and then try to decode what was read.

For python >= 3.5, don't fail when attempting to decode the
`diff_bytes` output in order to print it.

Finally, add some tests for encoding handling.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D68664

llvm-svn: 374389