Leading whitespace is trimmed from merge suggestions

Summary

Suggestions with leading whitespace seem to be applied incorrectly

Steps to reproduce

1. Comment with a multi-line suggestion on a merge request, that introduces some leading whitespace
2. Apply the suggestion using the UI

Example Project

https://gitlab.com/zevisert/2019-09-26-suggestion-issue/

Also the applied suggestion repro: https://gitlab.com/zevisert/2019-09-26-suggestion-issue/merge_requests/1#note_222854131

What is the current bug behavior?

Leading whitespace in the suggestion is removed, and preview line numbers don't align with the applied result

What is the expected correct behavior?

1. Preview shows the result we should get
2. Leading whitespace is not trimmed from suggestions

Relevant logs and/or screenshots

Here's the original issue we encountered, with some DOM slightly altered for privacy sake. Note the line numbers of the suggestion preview, and the current line numbers looking at the file, having the suggestion applied.

Output of checks

This bug happens on GitLab.com Also on gitlab-ee omibus installations

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Ubuntu 18.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.6.3p62
Gem Version:    2.7.9
Bundler Version:1.17.3
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.21.0
Sidekiq Version:5.2.7
Go Version:     unknown
GitLab information
Version:        12.1.6-ee
Revision:       d05ee0a9c12
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     10.7
URL:            https://git.[sanitized]
HTTP Clone URL: https://git.[sanitized]/some-group/some-project.git
SSH Clone URL:  git@git.[sanitized]:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:
GitLab Shell
Version:        9.3.0
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

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 9.3.0 ? ... OK (9.3.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: 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? ... yes
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: ...
5/1 ... yes
5/3 ... yes
5/4 ... yes
7/5 ... yes
7/6 ... yes
6/7 ... yes
6/8 ... yes
6/9 ... yes
5/10 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.3)
Git version >= 2.21.0 ? ... yes (2.21.0)
Git user has default SSH configuration? ... yes
Active users: ... 7
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished