Improve cached content header of status output

Improve cached content header of status output

This tries to be more to the point while also including a pointer on how to
unstage changes from the index.

Since this header is printed in two different code paths and the name of the
reference commit is needed for the unstage part, provide a new printing
function.

Signed-off-by: Jürgen Rühle <redacted>
Signed-off-by: Junio C Hamano <redacted>