This bug caused broken tree objects to get built by a later git annex sync.
This is a somewhat unlikely but not impossible situation, and the test
suite's union_merge_regression test tickled it when it was run on FAT.
Mostly the username is only used for the git committer or other display
purposes, and we can just fall back to a dummy value in these cases.
The only remaining place where an error is thrown is when starting local
pairing, which needs the username to be known.
I noticed move --to failing when there was no disk space. The file was sent
to the remote, but it crashed before it could be dropped locally. This
could fix that.
The queue could potentially contain changes from before withAltRepo, and
get flushed inside the call, which would apply the changes to the modified
repo.
Or, changes could be queued in withAltRepo that were intended to affect
the modified repo, but don't get flushed until later.
I don't know of any cases where either happens, but better safe than sorry.
Note that this affect withIndexFile, which is used in git-annex branch
updates. So, it potentially makes things slower. Should not be by much;
the overhead consists only of querying the current queue a couple of times,
and potentially flushing changes queued within withAltRepo earlier, that
could have maybe been bundled with other later changes.
Notice in particular that the existing queue is not flushed when calling
withAltRepo. So eg when git annex add needs to stage files in the index,
it will still bundle them together efficiently.
Added guard in Annex.Transfer to prevent this problem at a deeper level.
I'm unhappy ith NoUUID, but having Maybe UUID instead wouldn't help either
if nothing checked that there was a UUID. Since there legitimately need to
be Remotes that do not have a UUID, I can't see a way to fix it at the type
level, short making there be two separate types of Remotes.
