Paginator: Eagerly walk to the first matching commit

This allows getPreviousStart() to be called before the entire walk is
done, which will be necessary for streaming log results. It does mean
the Paginator constructor may block for a while, but we never
construct a Paginator without more or less immediately iterating
through it, so this difference is not noticeable.

Change-Id: Ie40c77678df4e2c3192c1c1fcc16463be57d5efc
1 file changed