manifest: use public API for obtaining storage object
We should not be accessing private attributes on the manifestlog,
since its behavior is governed by an interface.
Differential Revision: https://phab.mercurial-scm.org/D4392
perf: add function for obtaining manifest revision
This will use the modern API if available or fall back to the
legacy direct revlog access case. Without this, the perf command
won't work on all code bases.
Differential Revision: https://phab.mercurial-scm.org/D4391