[RFC] Reporting index properties

Heya,

As part of gathering some automated performance statistics of git, it would be useful to be able to extract some vital properties of the index. While `git update-index` has ways to _set_ the index version, and enable/disable various extensions, AFAICT there is no method by which one can ask for reporting about the current state of them -- short of re-implementing all of the index-parsing logic external to Git, which is not terribly appealing.

I hesitate to propose adding a flag to `git update-index` which reads but does no updating, as that seems to make a misnomer of the command. But this also doesn't seem worthy of a new top-level command.

Do folks have feelings about surfacing this information, and where such logic should live?

 - Alex