xdiff_set_find_func() is used to set user defined regular expressions
for finding function signatures.  Add xdiff_clear_find_func(), which
frees the memory allocated by the former, making the API complete.

Also, use the new function in diff.c (the only call site of
xdiff_set_find_func()) to clean up after ourselves.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>