revlog: improve documentation of the entry tuple
The code in revlog, and outside revlog directly use the index's entry tuple,
with direct integer indexing. This is a voluntary trade off to obtains better
performance from the Python code at the expense of the developers sanity.
Let's at least have a clear and central documentation about what this tuple is
about.
Differential Revision: https://phab.mercurial-scm.org/D10643
#require black test-repo
Black needs the real USERPROFILE in order to run on Windows
#if msys
$ USERPROFILE="$REALUSERPROFILE"
$ export USERPROFILE
#endif
$ cd $RUNTESTDIR/..
$ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`