Files can be left in .git/annex/misctmp when git-annex is interrupted in
the middle of an operation that uses it. Especially with `git annex add`
being interrupted can leave a large file hard linked in there, which can
become a problem for the user.

While it would be possible to hook SIGINT and clean up the files, that
wouldn't catch every way git-annex could be interrupted; ie power failure.

The assistant has some code that deletes misctmp files there older than 1
day. Perhaps when git-annex uses the misctmp directory it should first
perform such a cleanup pass.

A 1 day timeout is not ideal.. Imagine a git-annex add of a really big file
where the checksum takes more than 1 day.

Let's use a lock file. There can be a single one for the whole misctmp
directory. When it's being used, a shared lock is held. Take an exclusive
lock before cleaning it.

Since old versions of git-annex could still be in use and using the misctmp
directory, let's rename the directory that new versions of git-annex
use to .git/annex/othertmp.

[[done]] --[[Joey]]
