discovery: don't compute allfuturecommon when it won't be used In repos with many changesets, the computation of allfuturecommon can take a significant amount of time. Since it's only used if there's an obsstore, don't compute it otherwise.

treemanifest: separate flags for trees in memory and trees on disk When we start writing tree manifests with one manifest revlog per directory, it will still be nice to be able to run tests using tree manifests in memory but writing to a flat manifest to a single revlog. Let's break the current '_usetreemanifest' flag on the revlog into '_treeinmem' and '_treeondisk'. Both are populated from the same config, but after this change, one can temporarily hard-code _treeinmem=True to see that tests still pass.

manifestdict: drop empty-string argument when creating empty manifest manifestdict() creates an empty manifestdict, so let's consistently use that instead of explicitly parsing an empty string (which does result in an empty manifest).