rust: Make private the `index` field of the `Revlog` struct
To replace the previous use of this field from another module, add a
`node_from_rev` method. This is the same method that already existed on
`Changelog`.
Differential Revision: https://phab.mercurial-scm.org/D11414
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors