Add support for PreReceiveError in UserMergeBranch RPC

Closes gitaly#979

GITALY_SERVER_VERSION

-0.75.0
+0.76.0

Gemfile

@@ -406,7 +406,7 @@ group :ed25519 do
 end
 
 # Gitaly GRPC client
-gem 'gitaly-proto', '~> 0.82.0', require: 'gitaly'
+gem 'gitaly-proto', '~> 0.83.0', require: 'gitaly'
 
 gem 'toml-rb', '~> 0.3.15', require: false
 

Gemfile.lock

@@ -285,7 +285,7 @@ GEM
       po_to_json (>= 1.0.0)
       rails (>= 3.2.0)
     gherkin-ruby (0.3.2)
-    gitaly-proto (0.82.0)
+    gitaly-proto (0.83.0)
       google-protobuf (~> 3.1)
       grpc (~> 1.0)
     github-linguist (4.7.6)
@@ -1056,7 +1056,7 @@ DEPENDENCIES
   gettext (~> 3.2.2)
   gettext_i18n_rails (~> 1.8.0)
   gettext_i18n_rails_js (~> 1.2.0)
-  gitaly-proto (~> 0.82.0)
+  gitaly-proto (~> 0.83.0)
   github-linguist (~> 4.7.0)
   gitlab-flowdock-git-hook (~> 1.0.1)
   gitlab-markup (~> 1.6.2)

lib/gitlab/gitaly_client/operation_service.rb

@@ -103,7 +103,13 @@ module Gitlab
 
         request_enum.push(Gitaly::UserMergeBranchRequest.new(apply: true))
 
-        branch_update = response_enum.next.branch_update
+        second_response = response_enum.next
+
+        if second_response.pre_receive_error.present?
+          raise Gitlab::Git::HooksService::PreReceiveError, second_response.pre_receive_error
+        end
+
+        branch_update = second_response.branch_update
         return if branch_update.nil?
         raise Gitlab::Git::CommitError.new('failed to apply merge to branch') unless branch_update.commit_id.present?