git-commit: exit non-zero if we fail to commit the index

In certain rare cases, the creation of the commit object
and update of HEAD can succeed, but then installing the
updated index will fail. This is most likely caused by a
full disk or exceeded disk quota. When this happens the
new index file will be removed, and the repository will
be left with the original now-out-of-sync index. The
user can recover with a "git reset HEAD" once the disk
space issue is resolved.

We should detect this failure and offer the user some
helpful guidance.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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