userdiff: add xdiff_clear_find_func()

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: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1603,6 +1603,7 @@ static void builtin_diff(const char *name_a,
 			free(mf1.ptr);
 		if (textconv_two)
 			free(mf2.ptr);
+		xdiff_clear_find_func(&xecfg);
 	}
 
  free_ab_and_return: