changegroup: don't try to prune manifest nodes if not ellipses
In non-ellipses case, the number of manifest nodes can be very big, and finding
whether one of them can be pruned or not is very costly. For each node, we try
to find the rev and the linkrev, which is expensive.
Sending bit more manifest nodes seems better and much faster here. On our
internal repository, this saves around 5 seconds on `hg tracked --addinclude
<some_path>` on a narrow repo with ellipses disabled.
Differential Revision: https://phab.mercurial-scm.org/D5782