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

When checking out files, we're performing conversion into the user's
native line endings, but we only want to do it for files which have
consistent line endings. Refuse to perform the conversion for mixed-EOL
files.

The CRLF->LF filter is left as-is, as that conversion is considered to be
normalization by git and should force a conversion of the line endings.
Merge remote-tracking branch 'upstream/cmn/mixed-eol-passthrough'
Add CHANGELOG entry for mixed-EOL fix
git_blob_is_binary() calls git_buf_text_contains_nul()

Drop git_buf_text_is_binary()