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
@@ -43,13 +45,13 @@ def self.import!(rugged_commit)
   def self.new_from_rugged_commit(rugged_commit)
     new(
       sha1:            rugged_commit.oid,
-      author_name:     rugged_commit.author[:name].force_encoding('UTF-8'),
-      author_email:    rugged_commit.author[:email].force_encoding('UTF-8'),
+      author_name:     rugged_commit.author[:name].dup.force_encoding('UTF-8'),
+      author_email:    rugged_commit.author[:email].dup.force_encoding('UTF-8'),
       author_date:     rugged_commit.author[:time],
-      committer_name:  rugged_commit.committer[:name].force_encoding('UTF-8'),
-      committer_email: rugged_commit.committer[:email].force_encoding('UTF-8'),
+      committer_name:  rugged_commit.committer[:name].dup.force_encoding('UTF-8'),
+      committer_email: rugged_commit.committer[:email].dup.force_encoding('UTF-8'),
       committer_date:  rugged_commit.committer[:time],
-      message:         rugged_commit.message.force_encoding('UTF-8'),
+      message:         rugged_commit.message.dup.force_encoding('UTF-8'),
       merge:           rugged_commit.parents.size > 1
     )
   end
@@ -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'