treemanifest: store submanifest revlog per directory With this change, when tree manifests are enabled (in .hg/requires), commits will be written with one manifest revlog per directory. The manifest revlogs are stored in .hg/store/meta/$dir/00manifest.[id]. Flat manifests can still be read and interacted with as usual (they are also read into treemanifest instances). The functionality for writing treemanifest as a flat manifest to disk is still left in the code; tests still pass with '_treeinmem=True' hardcoded. Exchange is not yet implemented.