builtin/cat-file: mark 'git cat-file' sparse-index compatible

builtin/cat-file: mark 'git cat-file' sparse-index compatible

This change affects how 'git cat-file' works with the index when
specifying an object with the ":<path>" syntax (which will give file
contents from the index).

'git cat-file' expands a sparse index to a full index any time contents
are requested from the index by specifying an object with the ":<path>"
syntax. This is true even when the requested file is part of the sparse
index, and results in much slower 'git cat-file' operations when working
within the sparse index.

Mark 'git cat-file' as not needing a full index, so that you only pay
the cost of expanding the sparse index to a full index when you request
a file outside of the sparse index.

Add tests to ensure both that:
- 'git cat-file' returns the correct file contents whether or not the
  file is in the sparse index
- 'git cat-file' expands to the full index any time you request
  something outside of the sparse index

Signed-off-by: Kevin Lyles <klyles+github@epic.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>