webui can cause lakeFS to go over uncommitted deletes twice

[arielshaqed]

When displaying a branch, the objects page in the webui also computes and displays changes. To do this, it:

• Lists the objects on the branch.
• Lists the uncommitted changes.
• Merges the two lists to determine what to display.

Both of these listings need to scan the same KV entries, which is slow (and see below).

Instead we could:

• List the committed objects on the branch head (list branch@ instead of branch when branch is a branch; or ref~0 if ref@ gives us grief).
• List the uncommitted changes.
• Merge the two lists to determine what to display.

Now the first listing is committed-only, which makes it incredibly faster.

Why do we care? It's really slow, and this issue than doubles the effect of #2092!

[arielshaqed]

Relevant thread.