convert: disable current --filemap support
The --filemap support in hg convert doesn't handle merges correctly.
(And after
33015dac5df5 I managed to break it even for simple cases
where we don't want the first revision.)
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.
convert: pass the order of the revmapfile to the converter_source
The entries in this file are supposed to be topologically sorted and
this may be useful for a converter_source.