Fix path to mapmerge tool

Created by: Exxion

Wish I knew about this thing before my last/first PR. Judging by issue comments, this tool is used to merge .dmms, but nobody has updated .gitconfig in two years, so the driver has to be manually added to config because the file path for mapmerge.sh is outdated. Or something. Don't really know that much about Git at the moment. Feel free to scold me or something if this isn't actually helpful.