color: add color_parse_quietly()

color: add color_parse_quietly()

When parsing colors, a failed parse leads to an error message due to the
result returning error(). To allow for quiet parsing, create
color_parse_quietly(). This is in contrast to an ..._gently() version
because the original does not die(), so both options are technically
'gentle'.

To accomplish this, convert the implementation of color_parse_mem() into
a static color_parse_mem_1() helper that adds a 'quiet' parameter. The
color_parse_quietly() method can then use this. Since it is a near
equivalent to color_parse(), move that method down in the file so they
can be nearby while also appearing after color_parse_mem_1().

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>