The old lyric that goes “Everything old is new again” could easily be said for the production of “Hair: The American Tribal Love-Rock Musical” that can now be seen at the Mercury Theater, 3745 N. Southport Ave., through Sept. 17.With book and lyrics by Gerome Ragni and James Rado, “Hair” has music by Galt McDermot including outstanding songs such as “Aquarius,” “Good Morning Starshine,” “Easy to be Hard,” “I Got Life,” “Hair,&rdquo …