check_and_emit_line(): rename and refactor

check_and_emit_line(): rename and refactor

The function name was too bland and not explicit enough as to what it is
checking.  Split it into two, and call the one that checks if there is a
whitespace breakage "ws_check()", and call the other one that checks and
emits the line after color coding "ws_check_emit()".

Signed-off-by: Junio C Hamano <gitster@pobox.com>