diff-format.adoc: mode and hash are 0* for unmerged paths from index only

diff-format.adoc: mode and hash are 0* for unmerged paths from index only

In the "Raw output format" section, we mention that the 'mode' and
'sha1' for "src" and "dst" are 0* if "(creation|deletion) or unmerged".
For unmerged entries, 'mode' and 'sha1' are in fact 0* only when we are
looking at the index, i.e. on the left side for 'git diff-files' and on
the right side for 'git diff-index --cached'. Be more precise by
mentioning this, and while at it uniformize the wording of the "work
tree out of sync with the index" case.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>