Improve performance when looking up referenced objects.

Avoid a cat-file subprocess call per fat object blob by doing slightly
uglier parsing of "cat-file --batch" that includes object content,
instead of "cat-file --batch-check" that doesn't.

In my ad-hoc testing on a reasonable size repository
(50k rev-list -objects) this speeds up 'git fat status' by almost 40%.
More sophisticated use of cat-file bulk revision processing in two stages

This change takes better advantage of the relative strengths of
`cat-file --batch-check` and `cat-file --batch` and combines them.

* Uses `cat-file --batch-check` to filter the set of all git objects in
  bulk, leaving only those that are candidate git-fat object references,
  based on the object being of type "blob" and of magic size(s)
* Uses `cat-file --batch` to read the full contents of all the candidate
  objects in bulk, for fast processing of their data to find the actual
  git-fat references.
referenced_objects: make comment more precise
Merge branch 'jmurty/improve-referenced-objects-performance'

* jmurty/improve-referenced-objects-performance:
  referenced_objects: make comment more precise
  More sophisticated use of cat-file bulk revision processing in two stages
  Improve performance when looking up referenced objects.