Merge branch 'en/complete-sparse-checkout'

Command line completion (in contrib/) learned to complete path
arguments to the "add/set" subcommands of "git sparse-checkout"
better.

* en/complete-sparse-checkout:
  completion: avoid user confusion in non-cone mode
  completion: avoid misleading completions in cone mode
  completion: fix logic for determining whether cone mode is active
  completion: squelch stray errors in sparse-checkout completion