filemerge: call premerge directly from main merge function The merge code currently does (in pseudocode): for f in tomerge: premerge f merge f We'd like to change this to look more like: for f in tomerge: premerge f for f in tomerge: merge f This makes sure as many files are resolved as possible before prompting for the others. This restructuring is also necessary for custom merge drivers. This function separates out the premerge step from the merge step. In future patches we'll actually turn these into separate steps in the merge driver. The 'if r:' occurrences will be cleaned up in subsequent patches.