mergetools.rc: don't let meld open all changed files on startup
In meld 3.16, a multi-file change with option '-a' results in an overview list being opened. In meld 3.20, a multi-file change with option '-a' results in an overview list AND every changed file being opened.
Simply removing '-a' seems to work fine for both cases and also behaves the same as before for single-file changes.
As per the release notes, this is due to the following change in meld 3.19.0: "Make the --auto-compare command line flag work again (Kai Willadsen)"