Fix upgrading libdeps breaking stuff after cleanups

Fixes Homebrew/homebrew#2709.

By forcing dylibs to have an install_name id that is the HOMEBREW_PREFIX path, ie. the symlink’s path. Stuff that links to these dylibs will use this id and thus by immune to upgrades of underlying libraries.

Thus whatever keg is "current" ie. linked, will be the library that is used by the tool.

This fix is not retroactive. So there will still be breakage for existing installations of stuff.

The fix_install step in install is moved after the link step as the symlinking
is required to determine the eventual ids for each dylib.

Library/Homebrew/install.rb

@@ -119,16 +119,6 @@ def install f
 
   keg = Keg.new f.prefix
 
-  begin
-    keg.fix_install_names
-  rescue Exception => e
-    onoe "Failed to fix install names"
-    puts "The formula built, but you may encounter issues using it or linking other"
-    puts "formula against it."
-    ohai e, e.backtrace if ARGV.debug?
-    show_summary_heading = true
-  end
-
   begin
     require 'cleaner'
     Cleaner.new f if not f.pouring
@@ -204,6 +194,16 @@ def install f
       end
       show_summary_heading = true
     end
+
+    begin
+      keg.fix_install_names
+    rescue Exception => e
+      onoe "Failed to fix install names"
+      puts "The formula built, but you may encounter issues using it or linking other"
+      puts "formula against it."
+      ohai e, e.backtrace if ARGV.debug?
+      show_summary_heading = true
+    end
   end
 
   ohai "Summary" if ARGV.verbose? or show_summary_heading

Library/Homebrew/keg_fix_install_names.rb

@@ -2,6 +2,9 @@ class Keg
   def fix_install_names
     dylibs.each do |dylib|
       bad_install_names_for dylib do |id, bad_names|
+        # avoid the chmod change if unecessary—I'm not convinced it reverses right
+        next if bad_names.empty? and id.to_s == dylib.to_s
+
         dylib.ensure_writable do
           system "install_name_tool", "-id", id, dylib
           bad_names.each do |bad_name|
@@ -30,9 +33,11 @@ class Keg
     install_names.reject!{ |fn| fn =~ /^@(loader|executable)_path/ }
     install_names.reject!{ |fn| fn[0,1] == '/' }
 
-    unless install_names.empty? and id == dylib # avoid the work if possible
-      yield dylib, install_names
-    end
+    # the shortpath ensures that library upgrades don’t break installed tools
+    shortpath = HOMEBREW_PREFIX + Pathname.new(dylib).relative_path_from(self)
+    id = if shortpath.exist? then shortpath else dylib end
+
+    yield id, install_names
   end
 
   def dylibs