Fix squash rebase not working when diff contained encoded data

When the applied diff contains UTF-8 or some other encoded data, the diff
returned back from the git process may be in ASCII-8BIT format. Writing this
data to stdin may fail if the data because stdin expects this data to be in
UTF-8. By switching the output to binmode, we ensure that the diff will
always be written as-is.

Closes gitlab-org/gitlab-ee#4960

changelogs/unreleased/sh-fix-squash-rebase-utf8-data.yml

+---
+title: Fix squash rebase not working when diff contained encoded data
+merge_request:
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -2195,6 +2195,7 @@ module Gitlab
           # Apply diff of the `diff_range` to the worktree
           diff = run_git!(%W(diff --binary #{diff_range}))
           run_git!(%w(apply --index), chdir: squash_path, env: env) do |stdin|
+            stdin.binmode
             stdin.write(diff)
           end

spec/lib/gitlab/git/repository_spec.rb

+# coding: utf-8
 require "spec_helper"
 describe Gitlab::Git::Repository, seed_helper: true do
@@ -2221,6 +2222,17 @@ describe Gitlab::Git::Repository, seed_helper: true do
           subject
         end
       end
+
+      context 'with an ASCII-8BIT diff', :skip_gitaly_mock do
+        let(:diff) { "diff --git a/README.md b/README.md\nindex faaf198..43c5edf 100644\n--- a/README.md\n+++ b/README.md\n@@ -1,4 +1,4 @@\n-testme\n+✓ testme\n ======\n \n Sample repo for testing gitlab features\n" }
+
+        it 'applies a ASCII-8BIT diff' do
+          allow(repository).to receive(:run_git!).and_call_original
+          allow(repository).to receive(:run_git!).with(%W(diff --binary #{start_sha}...#{end_sha})).and_return(diff.force_encoding('ASCII-8BIT'))
+
+          expect(subject.length).to eq(40)
+        end
+      end
     end
   end