InstallRenamed: don't overwrite etc files; rename.

If an etc file exists on installation instead of overwriting it (or
requiring all the manual checks in formula) simply copy it with the
extension `.default` appended.

Library/Homebrew/extend/pathname.rb

@@ -50,6 +50,8 @@ class Pathname
     # and also broken symlinks are not the end of the world
     raise "#{src} does not exist" unless File.symlink? src or File.exist? src
 
+    dst = yield(src, dst) if block_given?
+
     mkpath
     if File.symlink? src
       # we use the BSD mv command because FileUtils copies the target and

Library/Homebrew/formula.rb

@@ -9,7 +9,7 @@ require 'build_environment'
 require 'build_options'
 require 'formulary'
 require 'software_spec'
-
+require 'install_renamed'
 
 class Formula
   include FileUtils
@@ -165,7 +165,13 @@ class Formula
   def kext_prefix; prefix+'Library/Extensions' end
 
   # configuration needs to be preserved past upgrades
-  def etc; HOMEBREW_GIT_ETC ? prefix+'etc' : HOMEBREW_PREFIX+'etc' end
+  def etc
+    etc = HOMEBREW_PREFIX+'etc'
+    etc = prefix+etc if HOMEBREW_GIT_ETC
+    etc.extend(InstallRenamed)
+    etc
+  end
+
   # generally we don't want var stuff inside the keg
   def var; HOMEBREW_PREFIX+'var' end
 

Library/Homebrew/install_renamed.rb

@@ -0,0 +1,17 @@
+module InstallRenamed
+  def install_p src, new_basename = nil
+    super do |src, dst|
+      dst += "/#{File.basename(src)}" if File.directory? dst
+      append_default_if_different(src, dst)
+    end
+  end
+
+  private
+
+  def append_default_if_different src, dst
+    if File.file? dst and !FileUtils.identical?(src, dst) and !HOMEBREW_GIT_ETC
+      dst += ".default"
+    end
+    dst
+  end
+end