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.