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.
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