Git: Update hook from `post-indexchanged` to `post-index-change`

[derrickstolee]

Git 2.22.0 includes the upstreamed version of the post-index-change hook. This requires some reaction work to rename the hook from our internal version.

A short-term fix is included in microsoft/git#140, but we will want to remove that quick fix with a more detailed fix in VFS for Git. It may require a disk layout version update.

[kewillford]

@derrickstolee why are you thinking this would require a disk layout version update?

[derrickstolee]

I guess we lay down the hooks at mount time? Then no update would be required. I guess I didn't think hard enough about the problem. If you're sure no upgrade steps are necessary, then we can just close this issue.

[derrickstolee]

Hook name updated by #1259.