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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.