Revert "Use to_s.start_with? in tag/branch ref method"

This reverts commit ec4730478b798270781257913ee4cede673d4d4e.

lib/gitlab/git.rb

@@ -54,11 +54,11 @@ module Gitlab
       end
 
       def tag_ref?(ref)
-        ref.to_s.start_with?(TAG_REF_PREFIX)
+        ref =~ /^#{TAG_REF_PREFIX}.+/
       end
 
       def branch_ref?(ref)
-        ref.to_s.start_with?(BRANCH_REF_PREFIX)
+        ref =~ /^#{BRANCH_REF_PREFIX}.+/
       end
 
       def blank_ref?(ref)