git.cp(): don't repeatedly recompute default repo

git.cp(): don't repeatedly recompute default repo

Before, bup would end up calling repo() for every invocation of cp().
Given that constructions like cp().get(id) are common, this could cause
a lot of unecessary calls, given that we already have git.repodir, which
will have the relevant value once initialized.

Discovered after noticing "bup restore" was hammering
"$BUP_DIR"/.git (which didn't exist).

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