Fix inconsistent lockfile order

When Gemfile would specify path sources as relative paths starting with
"./", the lockfile would have inconsistent order on `bundle install` and
`bundle update`.

lib/bundler/source/path.rb

@@ -24,7 +24,12 @@ def initialize(options)
 
         if options["path"]
           @path = Pathname.new(options["path"])
-          @path = expand(@path) unless @path.relative?
+          expanded_path = expand(@path)
+          @path = if @path.relative?
+            expanded_path.relative_path_from(root_path.expand_path)
+          else
+            expanded_path
+          end
         end
 
         @name    = options["name"]

spec/install/gemfile/path_spec.rb

@@ -83,6 +83,50 @@
     end
   end
 
+  it "sorts expanded paths consistently on install and update" do
+    build_lib "demo", :path => lib_path("demo")
+    build_lib "aaa", :path => lib_path("demo/aaa")
+
+    gemfile = <<-G
+      gemspec
+      gem "aaa", :path => "./aaa"
+    G
+
+    File.open(lib_path("demo/Gemfile"), "w") {|f| f.puts gemfile }
+
+    lockfile = <<~L
+      PATH
+        remote: .
+        specs:
+          demo (1.0)
+
+      PATH
+        remote: aaa
+        specs:
+          aaa (1.0)
+
+      GEM
+        specs:
+
+      PLATFORMS
+        ruby
+
+      DEPENDENCIES
+        aaa!
+        demo!
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    Dir.chdir(lib_path("demo")) do
+      bundle :install
+      expect(lib_path("demo/Gemfile.lock")).to have_lockfile(lockfile)
+      bundle :update, :all => true
+      expect(lib_path("demo/Gemfile.lock")).to have_lockfile(lockfile)
+    end
+  end
+
   it "expands paths when comparing locked paths to Gemfile paths" do
     build_lib "foo", :path => bundled_app("foo-1.0")