Summary:
When requesting trees from the server we try to provide a base tree
that we already have. In the hybrid manifest days, when we were fetching
manifest X, we would look up the linknode for X in the flat manifest revlog,
then scan the changelog up and down from that linknode. In the treeonly world we
don't have that revlog anymore though, so the current algorithm just searches from
the tip. This ended up being a really bad algorithm, since if you have tip and you
request tip~1000, then tip~2000, then tip~3000, you end up basing them all
against tip and redownloading a lot of data. This makes hg blame and hg log -p
super slow.
Let's change the algorithm to base off of a hint linknode, and let's change
changectx to set that hint every time it attempts to read a manifest. In manual
testing this significantly sped up hg log -p
Reviewed By: phillco
Differential Revision: D8399613
fbshipit-source-id: 771a94ee8b82be682ea0091b8d6c0fcd5f4e6646
