Merge branch 'ds/sparse-colon-path'

Merge branch 'ds/sparse-colon-path'

"git show :<path>" learned to work better with the sparse-index
feature.

* ds/sparse-colon-path:
  rev-parse: integrate with sparse index
  object-name: diagnose trees in index properly
  object-name: reject trees found in the index
  show: integrate with the sparse index
  t1092: add compatibility tests for 'git show'