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: 0:9426b370c206
tag: tip
user: My Name <myname@example.com>
date: Mon Jan 12 13:46:40 1970 +0000
summary: commit-1
changeset: 1:4997f15a1b24
tag: tip
user: foo@bar.com
date: Mon Jan 12 13:46:40 1970 +0000
summary: commit-1
changeset: 2:72b8012b424e
tag: tip
user: foobar <foo@bar.com>
date: Mon Jan 12 13:46:40 1970 +0000
summary: commit-1
changeset: 3:35ff3067bedd
tag: tip
user: foo@bar.com
date: Mon Jan 12 13:46:40 1970 +0000
summary: commit-1
transaction abort!
rollback completed
abort: Please specify a username.
No username found, using user@host instead