Merge on UI hangs if pre-receive Hook fails and the project's 'Merge method' is set to 'Fast-forward merge'

Summary

When a Gitlab project's 'Merge method' is set to 'Fast-forward merge' and the project has a pre-receive Hook, clicking the Merge button on a merge request will make the UI wait forever.

Steps to reproduce

We've installed a pre-receive Hook based on this: https://docs.gitlab.com/ee/administration/custom_hooks.html

The issue is reproducible with this very simple Hook:

#!/bin/sh

set -e

while read -r oldRevision newRevision refName; do

    echo "GL-HOOK-ERR: REJECTING: $oldRevision; $newRevision; $refName"
    exit 1

done

The Hook works perfectly when I try to push into the repository from a local clone, however when I click the Merge button on a merge request, the Merge button just hangs there.

I noticed that if I set the 'Merge method' to 'Merge commit' in the project's setting and click the Merge button, the message logged by the pre-receive Hook will be correctly displayed on the UI.

Using the latest community edition.

What is the current bug behavior?

Merge hangs if pre-receive Hook declines and the 'Merge method' is set to 'Fast-forward merge'.

What is the expected correct behavior?

UI should display the error logged by the pre-receive Hook.

Results of GitLab environment info

System information
System:		Ubuntu 18.04
Current User:	git
Using RVM:	no
Ruby Version:	2.6.3p62
Gem Version:	2.7.9
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	3.2.12
Git Version:	2.22.0
Sidekiq Version:5.2.7
Go Version:	unknown

GitLab information
Version:	12.5.2
Revision:	49482945d28
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	10.9
URL:		http://myHost:9674
HTTP Clone URL:	http://myHost:9674/some-group/some-project.git
SSH Clone URL:	git@myHost:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	10.2.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Checking GitLab subtasks ...

Checking GitLab Shell ...

GitLab Shell: ... GitLab Shell version >= 10.2.0 ? ... OK (10.2.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Gitaly ...

Gitaly: ... default ... OK

Checking Gitaly ... Finished

Checking Sidekiq ...

Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1

Checking Sidekiq ... Finished

Checking Incoming Email ...

Incoming Email: ... Reply by email is disabled in config/gitlab.yml

Checking Incoming Email ... Finished

Checking LDAP ...

LDAP: ... LDAP is disabled in config/gitlab.yml

Checking LDAP ... Finished

Checking GitLab App ...

Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... skipped (no tmp uploads folder yet)
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ... 
6/1 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.3)
Git version >= 2.22.0 ? ... yes (2.22.0)
Git user has default SSH configuration? ... yes
Active users: ... 3
Is authorized keys file accessible? ... yes

Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished