Merge branch 'ld/sparse-diff-blame'

Merge branch 'ld/sparse-diff-blame'

Teach diff and blame to work well with sparse index.

* ld/sparse-diff-blame:
  blame: enable and test the sparse index
  diff: enable and test the sparse index
  diff: replace --staged with --cached in t1092 tests
  repo-settings: prepare_repo_settings only in git repos
  test-read-cache: set up repo after git directory
  commit-graph: return if there is no git directory
  git: ensure correct git directory setup with -h