Merge branch 'jc/update-index-show-index-version'

"git update-index" learns "--show-index-version" to inspect the index format version used by the on-disk index file. * jc/update-index-show-index-version: test-tool: retire "index-version" update-index: add --show-index-version update-index doc: v4 is OK with JGit and libgit2