convert: encapsulate commit data fetching and commit object creation Split fetching the `describe` form from Perforce and the commit object creation into two functions. This allows us to reuse the commit construction for revisions passed from a revmap.

convert: do not provide head revisions if we have no changests to import Don't set a head revision in cases where we have a revmap but no changesets to import, as convertcmd.convert() treats them as heads of to-imported revisions.

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.

convert: use convert_revision for P4 imports We are using convert_revisions in other importers. In order to unify this we are also using convert_revision for Perforce in addition to the original 'p4'.

convert: remove unused dictionaries self.parent, self.lastbranch and self.tags have never been used.

convert: self.heads is a list self.heads is used as a list throughout convert and never a dictionary. Initialize it correctly to a list.

convert: don't use long list comprehensions We are iterating over p4changes. Make the continue condition more clear and easier to add new conditions in future patches, by removing the list comprehension and move the condition into the existing for-loop.