Synchronize Keg#link and #unlink counts

Keg#link would sometimes count a linked file when doing mkpath, even if
the target directory already exists; #unlink would never count it. This
meant that "brew ln" and "brew unlink" counts for the same keg could be
out of sync with each other.

Library/Homebrew/extend/pathname.rb

@@ -442,6 +442,7 @@ module ObserverPathnameExtension
     $d+=1
   end
   def mkpath
+    return if exist?
     super
     puts "mkpath #{to_s}" if ARGV.verbose?
     $d+=1