Don't run any git commands when sorting and comparing git sources

Previously, when sorting and comparing git Gemfile vs lockfile sources during
`bundler/setup` to figure out whether we need to re-resolve or not, we
would try to find the default branch if nothing more specific was
specified in the Gemfile.

If the git cache has been deleted thought, that would fail.

The error would still be swallowed (and the branch would simply not be
displayed), but trying to clone would still generate the side effect of
creating the parent folder for the clone.

That could affect non-writable systems that don't expect `bundler/setup`
to write to the filesystem at all.

To fix this, override `Bundler::Source::Git#identifier` to use
exclusively static information, so it does not even try to clone the
repo nor generate any side effects.

Author: deivid-rodriguez

bundler/lib/bundler/source/git.rb

@@ -69,19 +69,7 @@ def eql?(other)
 
       def to_s
         begin
-          at = if local?
-            path
-          elsif user_ref = options["ref"]
-            if /\A[a-z0-9]{4,}\z/i.match?(ref)
-              shortref_for_display(user_ref)
-            else
-              user_ref
-            end
-          elsif ref
-            ref
-          else
-            current_branch
-          end
+          at = humanized_ref || current_branch
 
           rev = "at #{at}@#{shortref_for_display(revision)}"
         rescue GitError
@@ -91,6 +79,10 @@ def to_s
         uri_with_specifiers([rev, glob_for_display])
       end
 
+      def identifier
+        uri_with_specifiers([humanized_ref, cached_revision, glob_for_display])
+      end
+
       def uri_with_specifiers(specifiers)
         specifiers.compact!
 
@@ -256,6 +248,20 @@ def local?
 
       private
 
+      def humanized_ref
+        if local?
+          path
+        elsif user_ref = options["ref"]
+          if /\A[a-z0-9]{4,}\z/i.match?(ref)
+            shortref_for_display(user_ref)
+          else
+            user_ref
+          end
+        elsif ref
+          ref
+        end
+      end
+
       def serialize_gemspecs_in(destination)
         destination = destination.expand_path(Bundler.root) if destination.relative?
         Dir["#{destination}/#{@glob}"].each do |spec_path|

bundler/spec/install/gemfile/git_spec.rb

@@ -30,6 +30,13 @@
       expect(Dir["#{default_bundle_path}/cache/bundler/git/foo-1.0-*"]).to have_attributes :size => 1
     end
 
+    it "does not write to cache on bundler/setup" do
+      cache_path = default_bundle_path.join("cache")
+      FileUtils.rm_rf(cache_path)
+      ruby "require 'bundler/setup'"
+      expect(cache_path).not_to exist
+    end
+
     it "caches the git repo globally and properly uses the cached repo on the next invocation" do
       simulate_new_machine
       bundle "config set global_gem_cache true"