COBranch: rename -makeCopyFromRevision: to -makePersistentRootCopyFromRevision:. Add -makePersistentRootCopy. Alter the argument check to be 'The revision must be equal to or an ancestor of 'headRevision'.', and document it.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10186 7b3f36aa-e6db-49c1-8b4c-66eac1997e64