filemerge: break overall filemerge into separate premerge and merge steps This means that in ms.resolve we must call merge after calling premerge. This doesn't yet mean that all premerges happen before any merges -- however, this does get us closer to our goal. The output differences are because we recompute the merge tool. The only user-visible difference caused by this patch is that if the tool is missing we'll print the warning twice. Not a huge deal, though.