Fixes bug that caused git-annex to fail to add a file when another
git-annex process cleaned up the temp directory it was using.
Solution is just to push withOtherTmp out to a higher level, so that
the whole ingest process can be completed inside it.
But in the assistant, that was not practical to do, since withOtherTmp runs
in the Annex monad and the assistant does not. Worked around by introducing
a separate temp directory that only the assistant uses for lockdown.
Since only one assistant can run at a time, it's easy to clean up that
directory of old cruft at startup.
This is only done for correctness sake; I don't see any way that it
would have caused a problem here. The jlog file escaped withOtherTmp
so another process could swoop in and delete it, but the file is only
used as a buffer for a list of filenames, and its handle gets rewound
and they're read back out, which will still work even if it's already
been deleted.
The only reason I didn't just pre-delete the file and keep the handle
open is I'm not sure that works on all OS's (eg Windows). If there was
a problem that this fixed it might involve an OS that doesn't support
deleting an open file or something like that.
