DEV: Enable unless cops

We discussed the use of `unless` internally and decided to enforce
available rules from rubocop to restrict its most problematic uses.

lib/onebox/mixins/git_blob_onebox.rb

@@ -120,15 +120,15 @@ module Onebox
           a_lines.each do |l|
             l = l.chomp("\n") # remove new line
             m = l.match(/\A[ ]*/) # find leading spaces 0 or more
-            unless m.nil? || l.size == m[0].size || l.size == 0 # no match | only spaces in line | empty line
+            if m.nil? || l.size == m[0].size || l.size == 0
+              next # SKIP no match or line is only spaces
+            else # no match | only spaces in line | empty line
               m_str_length = m[0].size
               if m_str_length <= 1 # minimum space is 1 or nothing we can break we found our minimum
                 min_space = m_str_length
                 break #stop iteration
               end
               min_space = m_str_length if m_str_length < min_space
-            else
-              next # SKIP no match or line is only spaces
             end
           end
           a_lines.each do |l|