Creates a
private branch revision off the specified
public trunk revision.
A {@code MicroKernelException} is thrown if {@code trunkRevisionId} doesn'texist, if it's not a
trunk revision (i.e. it's not reachable by traversing the revision history in reverse chronological order starting from the current head revision) or if another error occurs.
@param trunkRevisionId id of public trunk revision to base branch on,if {@code null} the current head revision is assumed
@return id of newly created private branch revision
@throws MicroKernelException if {@code trunkRevisionId} doesn't exist,if it's not a
trunk revision or if another error occurs
@see #merge(String,String)