Index is accessible, just like committed contents. The fact that gitk, qgit,
git-gui doesn't display state of index is their limitation.
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Actually qgit let's you see state index for each file when committing
with qgit commit dialog.

Indeed you can also choose to update only the index state of a file
without committing, ie without create a new tree object.

Internally, the index state of each file is known and tracked. No
other interface is provided, as example a diff, just because I found
it confusing and of little concrete help.

Of course if knowning the index state became important to perform some
concrete and quite common operation I could add whatever GUI would
suite.

Suggestions are welcomed ;-)