separate handles for cat-file and cat-file --batch-check

separate handles for cat-file and cat-file --batch-check

This avoids starting one process when only the other one is needed.
Eg in git-annex smudge --clean, this reduces the total number of
cat-file processes that are started from 4 to 2.

The only performance penalty is that when both are needed, it has to do
twice as much work to maintain the two Maps. But both are very small,
consisting of 1 or 2 items, so that work is negligible.

Sponsored-by: Dartmouth College's Datalad project