config: do not use strbuf_split()

config: do not use strbuf_split()

When parsing an old-style GIT_CONFIG_PARAMETERS environment
variable, the code parses key=value pairs by splitting them at '='
into an array of strbuf's.  As strbuf_split() leaves the delimiter
at the end of the split piece, the code has to manually trim it.

If we split with string_list_split(), that becomes unnecessary.
Retire the use of strbuf_split() from this code path.

Note that the max parameter of string_list_split() is of
an ergonomically iffy design---it specifies the maximum number of
times the function is allowed to split, which means that in order to
split a text into up to 2 pieces, you have to pass 1, not 2.

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