Note when --only-dependencies is a no-op

Library/Homebrew/formula_installer.rb

@@ -176,8 +176,13 @@ class FormulaInstaller
     check_requirements(req_map)
 
     deps = [].concat(req_deps).concat(f.deps)
+    deps = expand_dependencies(deps)
 
-    install_dependencies expand_dependencies(deps)
+    if deps.empty? and only_deps
+      puts "All dependencies for #{f} are satisfied."
+    else
+      install_dependencies(deps)
+    end
   end
 
   def check_requirements(req_map)