Initial backend implementation for 'tla' aka Arch 1.x
The backend maps the tailor repository, module and start-revision
options to 'tla' archive, category--branch--version and revision.
Current limitations and pitfalls.
Target backend not implemented.
In-version continuations not supported (raises an exception); this
would probably require to compute a changeset with 'tla delta'
instead of using replay.
Pika escaped file names. This implementations requires a version
of tla that supports pika escapes. For changesets created with a
version of tla that did not support pika escapes, if one of these
changeset contains a file name with a valid embedded pika escape
sequence, things will break.