There are some index handling subtleties in 'commit --only' that are
best tested when we have an existing index, but an unborn or empty
HEAD.  These circumstances are easily produced by 'checkout --orphan',
but we did not previously have a test for it.

The main expected failure mode would be: erroneously loading the
existing index contents when building the temporary index that is used
for --only.  Cf.

  http://article.gmane.org/gmane.comp.version-control.git/225969

and subsequent discussion.
Signed-off-by: NThomas Rast <trast@inf.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>