Currently, our object scanner is based on calling git cat-file --batch.
This is less efficient than reading the objects ourselves in process, so
update the object scanner to use our gitobj package instead.
Remove the reset function from the object scanner, since it is no longer
used. Also, since we no longer need to worry about reading too much
data from git cat-file --batch, switch the io.LimitedReader to a plain
io.Reader to simplify the code.
Note that in the tests we now specify the object ID of the pointer item
to scan since we no longer can implicitly read from a fake buffer.
Consequently, we no longer look for a trailing entry, since that won't
succeed anymore.
