merge: use string_list_split() in add_strategies()

merge: use string_list_split() in add_strategies()

Call string_list_split() for cutting a space separated list into pieces
instead of reimplementing it based on struct strategy.  The attr member
of struct strategy was not used split_merge_strategies(); it was a pure
string operation.  Also be nice and clean up once we're done splitting;
the old code didn't bother freeing any of the allocated memory.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>