perf: omit copying ui and redirect to ferr if buffer API is in use This allow to get the output of contrib/perf.py commands using the ui.pushbuffer() API.

manifest: change treemanifestctx to construct subtrees from the manifestlog Previously, treemanifestctx would directly construct its subtrees. By making it get the subtrees through manifestlog.get() we consolidate all treemanifestctx creation into manifestlog.get() and therefore extensions that need to wrap manifestctx creation (like narrow-hg) can intercept manifestctxs at that single place. This also means fetching subtrees will take advantage of the manifestlog ctx cache now, which it did not before.