convert: allow passing in a revmap Implement `common.setrevmap` which is used to pass in a file with existing revision mappings. This functionality is used by `convertcmd.convert` if it exists and allows implementors such as the p4 converter to make use of an existing mapping. We are using the revmap to abort scanning and the repository for more information if we already have the revision. This means we are allowing incremental imports in cases where a revmap is provided.