Skip UTF-8 BOM in binary detection

When a git_buf contains a UTF-8 BOM, the three bytes comprising
that BOM are treated as unprintable characters.  For a small git_buf,
the three BOM characters overwhelm the printable characters.  This
is problematic when trying to check out a small file as the CR/LF
filtering will not apply.
Merge pull request #1801 from ethomson/utf8_bom

Skip UTF-8 BOM in binary detection