test-cat-file.sh: handle hashsplit bupm files

A bit back we started allowing .bupm files to be hashsplit:

  ee777400cf624b4f7a24b15614c2b6c745560dcb

So add t/git-cat-tree and use it in test-cat-file.sh to prevent the
test from crashing whenever the bupm is a tree.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>