Try really hard to load a commit or tag

Try really hard to load a commit or tag

When we need the canonical form of a commit or a tag in order to
parse it into our RevCommit or RevTag fields, we really need it as a
single contiguous byte array.  However the ObjectDatabase may choose
to give us a large loader.  In general commits or tags are always
under the several MiB limit, so even if the loader calls it "large"
we should still be able to afford the JVM heap memory required to
get a single byte array.  Coerce even large loaders into a single
byte array anyway.

Change-Id: I04efbaa7b31c5f4b0a68fc074821930b1132cfcf
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>