Don't error if exact link already exists

If the link already exists exactly (well almost exactly) as we are about to correct it, then it's okay. Otherwise we error out. This is a safe choice, and really, the correct choice too.

This will prevent the tickets like Homebrew/homebrew#11050 from occurring.

Library/Homebrew/cmd/link.rb

@@ -10,6 +10,11 @@ module Homebrew extend self
     end
 
     ARGV.kegs.each do |keg|
+      if keg.linked_keg_record.directory? and keg.linked_keg_record.realpath == keg
+        opoo "Already linked: #{keg}"
+        next
+      end
+
       print "Linking #{keg}... " do
         puts if ARGV.verbose?
         puts "#{keg.link} symlinks created"

Library/Homebrew/keg.rb

@@ -102,7 +102,7 @@ class Keg < Pathname
       end
     end
 
-    (HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname).make_relative_symlink(self)
+    linked_keg_record.make_relative_symlink(self)
 
     return $n + $d
   end
@@ -123,6 +123,14 @@ protected
     puts "Won't resolve conflicts for symlink #{dst} as it doesn't resolve into the Cellar" if ARGV.verbose?
   end
 
+  def make_relative_symlink dst, src
+    if dst.exist? and dst.realpath == src.realpath
+      puts "Skipping; already exists: #{dst}" if ARGV.verbose?
+    else
+      dst.make_relative_symlink src
+    end
+  end
+
   # symlinks the contents of self+foo recursively into /usr/local/foo
   def link_dir foo
     root = self+foo
@@ -141,10 +149,10 @@ protected
         when :skip_file
           Find.prune
         when :info
-          dst.make_relative_symlink(src)
+          make_relative_symlink dst, src
           dst.install_info
         else
-          dst.make_relative_symlink(src)
+          make_relative_symlink dst, src
         end
       elsif src.directory?
         # if the dst dir already exists, then great! walk the rest of the tree tho
@@ -161,7 +169,7 @@ protected
           dst.mkpath unless resolve_any_conflicts(dst)
         else
           unless resolve_any_conflicts(dst)
-            dst.make_relative_symlink(src)
+            make_relative_symlink dst, src
             Find.prune
           end
         end