They're only needed to cover a gc edge case, and it's better someone
gets caught by that edge case than that someone who does not know about
them ends up with a filtered git-annex branch that contains such a tree
when some of the files listed in it are ones they wanted to *remove*
from the repository.
It's not currently possible to exclude a sameas repo using its
annex-config-uuid. (Remote.nameToUUID rejects them).
Since there's no real documented way to learn those, this seems ok, at
least for now. Also it avoids the problem of someone excluding the
parent but including the sameas, which would probably make the sameas
repo not usable when using the filtered branch.
Added a note to man page about what happens to information that is
recorded in the private journal. Since it uses Branch.get, that
information will be copied when options allow. It seemed better to allow
it and document it than not allow it, since the options allow excluding
repositories and so can be used to exclude private repos if desired.
Not tested yet but should work.
Noted a possible optimisation, which should probably be added, to
speed it up in cases where there is no uuid filtering being done.
It would need Annex.Branch to add a function like getRef that uses
catFileDetails, so the sha is also returned. The difficulty would be
making it support the precached file content; if it didn't it would
probably not be any faster and could even be slower. So probably the
precaching would need to be changed to also cache the sha.
This is less erorr-prone, and easier for the user to reason about; it
preserves the man page's promise that only explicitly included
information will be copied.
