Fix conflict resolve to handle FailedPrecondition error correctly

Before this change, `Gitlab::Git::Conflict::Resolver#conflicts`
is meant to rescue `GRPC::FailedPrecondition` exception and re-raise
`Gitlab::Git::Conflict::Resolver::ConflictSideMissing`.

That wasn't working though as `wrapped_gitaly_errors` will rescue
it as a `GRPC::BadStatus` exception because that's the parent of
`GRPC::FailedPrecondition`.

That results to MR head diff not showing up when there are
conflicts with missing side.

This fix moves the rescue inside the `wrapped_gitaly_errors`
block so we can rescue appropriately.

Changelog: fixed

lib/gitlab/git/conflict/resolver.rb

@@ -18,9 +18,9 @@ module Gitlab
         def conflicts
           @conflicts ||= wrapped_gitaly_errors do
             gitaly_conflicts_client(@target_repository).list_conflict_files.to_a
-          end
           rescue GRPC::FailedPrecondition => e
             raise Gitlab::Git::Conflict::Resolver::ConflictSideMissing, e.message
+          end
         rescue GRPC::BadStatus => e
           raise Gitlab::Git::CommandError, e
         end

spec/lib/gitlab/git/conflict/resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Git::Conflict::Resolver do
+  let(:repository) { instance_double(Gitlab::Git::Repository) }
+  let(:our_commit_oid) { 'our-commit-oid' }
+  let(:their_commit_oid) { 'their-commit-oid' }
+  let(:gitaly_conflicts_client) { instance_double(Gitlab::GitalyClient::ConflictsService) }
+
+  subject(:resolver) { described_class.new(repository, our_commit_oid, their_commit_oid) }
+
+  describe '#conflicts' do
+    before do
+      allow(repository).to receive(:gitaly_conflicts_client).and_return(gitaly_conflicts_client)
+    end
+
+    it 'returns list of conflicts' do
+      conflicts = [double]
+
+      expect(gitaly_conflicts_client).to receive(:list_conflict_files).and_return(conflicts)
+      expect(resolver.conflicts).to eq(conflicts)
+    end
+
+    context 'when GRPC::FailedPrecondition is raised' do
+      it 'rescues and raises Gitlab::Git::Conflict::Resolver::ConflictSideMissing' do
+        expect(gitaly_conflicts_client).to receive(:list_conflict_files).and_raise(GRPC::FailedPrecondition)
+        expect { resolver.conflicts }.to raise_error(Gitlab::Git::Conflict::Resolver::ConflictSideMissing)
+      end
+    end
+  end
+end