rbinstall.rb: drive letter in without_destdir

* tool/rbinstall.rb (without_destdir): compare with the destdir
  after stripping a drive letter, on dosish platforms.
  pointed out by @DavidEGrayson.
  https://github.com/ruby/ruby/commit/d0cf23b#commitcomment-16100407

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53838 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,10 @@
+Tue Feb 16 02:49:41 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* tool/rbinstall.rb (without_destdir): compare with the destdir
+	  after stripping a drive letter, on dosish platforms.
+	  pointed out by @DavidEGrayson.
+	  https://github.com/ruby/ruby/commit/d0cf23b#commitcomment-16100407
+
 Mon Feb 15 15:44:09 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (parse_ident): allow keyword arguments just after a

tool/rbinstall.rb

@@ -281,9 +281,9 @@ def with_destdir(dir)
 end
 
 def without_destdir(dir)
-  return dir if !$destdir or $destdir.empty? or !dir.start_with?($destdir)
+  return dir if !$destdir or $destdir.empty?
   dir = dir.sub(/\A\w:/, '') if File::PATH_SEPARATOR == ';'
-  dir[$destdir.size..-1]
+  dir.start_with?($destdir) ? dir[$destdir.size..-1] : dir
 end
 
 def prepare(mesg, basedir, subdirs=nil)