nodemap: keep track of the docket for loaded data
To perform incremental update of the on disk data, we need to keep tracks of
some aspect of that data.
Differential Revision: https://phab.mercurial-scm.org/D7885
#require black $ cd $RUNTESTDIR/.. $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`