We weren't using it, and in retrospect, it makes no sense to use
this API cat_file for giant responses which can't read quickly
with minimal context-switching (or sanely fit into memory for
Email::Simple/Email::MIME).
For giant blobs which we don't want slurped in memory, we'll
spawn a short-lived git-cat-file process like we do in ViewVCS.
Otherwise, monopolizing a git-cat-file process for a giant
blob is harmful to other PSGI/NNTP users.
A better interface is coming which will be more suitable for
for batch processing of "small" objects such as commits and
email blobs.