Fuzz tests have shown that git cat-file --batch sometimes stops running.
It's not yet known why (no error message; repo seems ok). But this is
something we can deal with in the CoProcess framework, since all 3 types of
long-running git processes should be restartable if they fail.
Note that, as implemented, only IO errors are caught. So an error thrown
by the reveiver, when it sees something that is not valid output from
git cat-file (etc) will not cause a restart. I don't want it to retry
if git commands change their output or are just outputting garbage.
This does mean that if the command did a partial output and crashed in the
middle, it would still not be restarted.
There is currently no guard against restarting a command repeatedly, if,
for example, it crashes repeatedly on startup.
The list of files had to be retained until the end so it could be deleted.
Also, a list of update-index lines was generated and only then fed into it.
Now everything streams in constant space.
When hashing the files, the entire list of shas was read strictly.
That was entirely unnecessary, since there's a cleanup action run
after they're consumed.
Could have just used hGetContentsStrict here, but that would require
storing all the shas in memory. Since this is called at the end of a
git-annex run, it may have created a *lot* of shas, so I avoid that memory
use and stream them out like before.
