Point at which CVS was made read-only.

May be useful if you have diffs in an old CVS checkout from
cvs.sv.gnu.org:/sources/m4, and want to import them into a
git checkout.  Try the following to import those diffs into git:

git checkout -b from-cvs cvs-readonly
cp -R path/to/old-cvs-tree/m4/* .
git commit -a -m 'state of old-cvs-tree'
git rebase master