Avoid unbounded getCachedBytes during parseAny

Avoid unbounded getCachedBytes during parseAny

Since we don't know the type of object we are parsing, we don't
know if its a massive blob, or some small commit or annotated tag.
Avoid pulling the cached bytes until we have checked the type and
decided if we actually need them to continue parsing right now.

This way large blobs which won't fit in memory and would throw
a LargeObjectException don't abort parsing.

Change-Id: Ifb70df5d1c59f616aa20ee88898cb69524541636
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>