Fix warning with frozen string

Author: rafaelfranca

app/models/commit.rb

@@ -1,3 +1,5 @@
+# frozen-string-literal: true
+
 class Commit < ApplicationRecord
   has_many :contributions, dependent: :destroy
   has_many :contributors, through: :contributions
@@ -196,7 +198,7 @@ def cache_diff(repo)
   # git show, and is an expensive operation. So, we do this only for those
   # commits where this is needed, and cache the result in the database.
   def extract_changelog
-    changelog = ''
+    changelog = +''
     in_changelog = false
     diff.each_line do |line|
       if line =~ /^diff --git/

test/models/commit_test.rb

@@ -1,3 +1,5 @@
+# frozen-string-literal: true
+
 require 'test_helper'
 require 'ostruct'
 
@@ -22,7 +24,7 @@ def test_import
     tcomm = Time.current
     tauth = 1.day.ago
 
-    message = <<-MSG.strip_heredoc
+    message = +<<-MSG.strip_heredoc
       \u{1f4a3}
 
       We are relying on hash inequality in tests
@@ -32,8 +34,8 @@ def test_import
       sha1 = "b5ed79468289c15a685a82694dcf1adf773c91d#{i}"
       rugged_commit = OpenStruct.new
       rugged_commit.oid       = sha1
-      rugged_commit.author    = {name: 'Juanjo', email: '[email protected]', time: tauth}
-      rugged_commit.committer = {name: 'David', email: '[email protected]', time: tcomm}
+      rugged_commit.author    = {name: +'Juanjo', email: +'[email protected]', time: tauth}
+      rugged_commit.committer = {name: +'David', email: +'[email protected]', time: tcomm}
       rugged_commit.message   = message
       rugged_commit.parents   = parents