Fix UTF-8 conversion issues when resolving conflicts

Similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/63030,
when the commit message in the /resolve_conflicts endpoint contains a
UTF-8 character, the conversion to the Gitaly ASCII-8BIT value may flag
an error. To fix this, we run `force_encoding` on the commit message.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63062

changelogs/unreleased/sh-fix-utf-8-encoding-resolve-conflicts.yml

+---
+title: Fix UTF-8 conversion issues when resolving conflicts
+merge_request: 29453
+author:
+type: fixed

lib/gitlab/gitaly_client/conflicts_service.rb

@@ -67,7 +67,7 @@ module Gitlab
           their_commit_oid: @their_commit_oid,
           source_branch: source_branch,
           target_branch: target_branch,
-          commit_message: resolution.commit_message,
+          commit_message: encode_binary(resolution.commit_message),
           user: Gitlab::Git::User.from_gitlab(resolution.user).to_gitaly
         )
       end

spec/lib/gitlab/gitaly_client/conflicts_service_spec.rb

@@ -35,7 +35,7 @@ describe Gitlab::GitalyClient::ConflictsService do
     end
     let(:source_branch) { 'master' }
     let(:target_branch) { 'feature' }
-    let(:commit_message) { 'Solving conflicts' }
+    let(:commit_message) { 'Solving conflicts\n\nTést' }
     let(:resolution) do
       Gitlab::Git::Conflict::Resolution.new(user, files, commit_message)
     end
@@ -51,6 +51,18 @@ describe Gitlab::GitalyClient::ConflictsService do
       subject
     end
 
+    it 'handles commit messages with UTF-8 characters' do
+      allow(::Gitlab::GitalyClient).to receive(:call).and_call_original
+      expect(::Gitlab::GitalyClient).to receive(:call).with(anything, :conflicts_service, :resolve_conflicts, any_args) do |*args|
+        # Force the generation of request messages by iterating through the enumerator
+        args[3].to_a
+
+        double(resolution_error: nil)
+      end
+
+      subject
+    end
+
     it 'raises a relevant exception if resolution_error is present' do
       expect_any_instance_of(Gitaly::ConflictsService::Stub).to receive(:resolve_conflicts)
         .with(kind_of(Enumerator), kind_of(Hash)).and_return(double(resolution_error: "something happened"))