[rubygems/rubygems] Fix some failing Bundler tests with old Git.

Use the `git branch --list` rather than the `git branch -l` for better
compatibility. Because the `git branch -l` is used to create a new branch in
Git version < 2.20.0.

rubygems/rubygems@eac5be7d06

spec/bundler/support/builders.rb

@@ -554,7 +554,7 @@ def _build(options)
           raise "You can't specify `master` as the branch" if branch == "master"
           escaped_branch = Shellwords.shellescape(branch)
 
-          if @context.git("branch -l #{escaped_branch}", libpath).empty?
+          if @context.git("branch --list #{escaped_branch}", libpath).empty?
             @context.git("branch #{escaped_branch}", libpath)
           end