string-list: unify string_list_split* functions

string-list: unify string_list_split* functions

Thanks to the previous step, the only difference between these two
related functions is that string_list_split() works on a string
without modifying its contents (i.e. taking "const char *") and the
resulting pieces of strings are their own copies in a string list,
while string_list_split_in_place() works on a mutable string and the
resulting pieces of strings come from the original string.

Consolidate their implementations into a single helper function, and
make them a thin wrapper around it.  We can later add an extra flags
parameter to extend both of these functions by updating only the
internal helper function.

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