diff: simplify parsing of diff.colormovedws

diff: simplify parsing of diff.colormovedws

The code to parse this configuration variable, whose value is a
comma-separated list of known tokens like "ignore-space-change" and
"ignore-all-space", uses string_list_split() to split the value into
pieces, and then places each piece of string in a strbuf to trim,
before comparing the result with the list of known tokens.

Thanks to the previous steps, now string_list_split() can trim the
resulting pieces before it places them in the string list.  Use it
to simplify the code.

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