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.

filemerge: only copy to backup during premerge step The premerge might leave the original file in an unclean state. Therefore it's important to only copy the file in the beginning.

filemerge: only print out "merging f" output at premerge step We're soon going to call this function twice, once for premerge and once for merge. This makes sure the "merging" output only gets printed during the premerge step.

filemerge: deindent the parts of filemerge outside the try block It is no longer necessary to indent these parts.

filemerge: introduce a premerge flag and function This flag will let us get to our overall goal of performing all premerges before any merges.

filemerge: also return whether the merge is complete In future patches, we'll pause merges after the premerge step. After the premerge step we'll return complete = False.