convert: allow the converter_source to say "skip this revision" If getchanges returns a string, it's assumed to be the id of an already converted revision. We map the current revision to the same revision this converted revision was mapped to. To allow skipping a root revision, getchanges can return the special string 'hg-convert-skipped-revision' (a.k.a. common.SKIPREV), which hopefully won't clash with any real id. The converter_source is responsible for rewriting the parents of the commit objects to make sure the revision graph makes sense.

changeset:   0:9c2cf2b35aa7
user:        test
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     add foo

changeset:   1:3756a9556b89
tag:         tip
user:        test
date:        Mon Jan 12 13:46:41 1970 +0000
summary:     change foo

2 revisions:
9c2cf2b35aa7
3756a9556b89