James' answer was the best, but let me add:1. Should I use WINE git? if so, is there any guide?
2. When I compile WINE's code, must I wait 30 minutes for each
compilation, or will it update only the modified files?
3. Once I've fixed (or at least spotted) the problem, how do I make it
available for others to test and apply it into official WINE?
1. Yes, http://wiki.winehq.org/GitWine
2. It updates only the modified files
3. You send a patch to wine-patches
But don't expect it to be easy, and don't forget to write a
conformance test, if possible.