detect write failure, even for stdout

git doesn't always detect write failures.  A write I/O error,
(e.g., hardware I/O error or simply disk full)
doesn't provoke nonzero exit status:

    $ ./git-cat-file -t HEAD > /dev/full && echo did not detect write failure
    did not detect write failure

This is perhaps more important than the other things
I've reported, since it can lead to porcelain being unable
to detect a real failure in the plumbing.

Here are two more:

    $ ./git-ls-tree HEAD > /dev/full && echo fail
    fail
    $ ./git-show > /dev/full && echo fail
    fail

If you were using gnulib, I'd suggest simply adding this line

    atexit (close_stdout);

near the beginning of each `main'.  Then you wouldn't have to
manually track down each and every place where a write to stdout
can occur -- not to mention the maintenance burden of keeping
things correct as the code evolves.