xdiff: drop unused mmfile parameters from xdl_do_patience_diff()

The entry point to the patience-diff algorithm takes two mmfile_t structs with the original file contents, but it doesn't actually do anything useful with them. This is similar to the case recently cleaned up in the histogram code via f1d019071e (xdiff: drop unused mmfile parameters from xdl_do_histogram_diff(), 2022-08-19), but there's a bit more subtlety going on. We pass them into the recursive patience_diff(), which in turn passes them into fill_hashmap(), which stuffs the pointers into a struct. But the only thing which reads the struct fields is our recursion into patience_diff()! So it's unlikely that something like -Wunused-parameter could find this case: it would have to detect the circular dependency caused by the recursion (not to mention tracing across struct field assignments). But once found, it's easy to have the compiler confirm what's going on: 1. Drop the "file1" and "file2" fields from the hashmap struct definition. Remove the assignments in fill_hashmap(), and temporarily substitute NULL in the recursive call to patience_diff(). Compiling shows that no other code touched those fields. 2. Now fill_hashmap() will trigger -Wunused-parameter. Drop "file1" and "file2" from its definition and callsite. 3. Now patience_diff() will trigger -Wunused-parameter. Drop them there, too. One of the callsites is the recursion with our NULL values, so those temporary values go away. 4. Now xdl_do_patience_diff() will trigger -Wunused-parameter. Drop them there. And we're done. Suggested-by: Phillip Wood <phillip.wood@dunelm.org.uk> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>