Merge branch 'fix-gh-pull-requests' into 'master'

Fix attr reader to force the intended values for source and target shas

## What does this MR do?

When importing a pull request from GitHub, the old and new branches may no longer actually exist by those names, but we need to recreate the merge
request diff with the right source and target shas.  We use these `target_branch_sha` and `source_branch_sha` attributes to force  these to the intended values. But the reader methods were always looking up to the target/source branch head instead of check if these values was  previously set, this MR applies a memoization pattern to both of them.

## Are there points in the code the reviewer needs to double check?

This [commit](https://gitlab.com/gitlab-org/gitlab-ce/commit/6ce25e7b4caa9e94de74378729178c7060d640b2) introduced this bug in the importer.

## What are the relevant issue numbers?

gitlab-org/gitlab-ce#20385

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [X] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [X] ~~API support added~~
- Tests
  - [X] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

/cc @akitaonrails @DouweM 

See merge request !5573

3 files changed, 18 insertions, 5 deletions

diff --git a/CHANGELOG b/CHANGELOG
index b54d9bbe733..39b77460deb 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -43,6 +43,7 @@ v 8.11.0 (unreleased)
   - Sensible state specific default sort order for issues and merge requests !5453 (tomb0y)
 
 v 8.10.3 (unreleased)
+  - Fix importer for GitHub Pull Requests when a branch was removed
   - Fix hooks missing on imported GitLab projects
   - Properly abort a merge when merge conflicts occur
   - Ignore invalid IPs in X-Forwarded-For when trusted proxies are configured.
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 471e32f3b60..f1b9c1d6feb 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -238,11 +238,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def target_branch_sha
-    target_branch_head.try(:sha)
+    @target_branch_sha || target_branch_head.try(:sha)
   end
 
   def source_branch_sha
-    source_branch_head.try(:sha)
+    @source_branch_sha || source_branch_head.try(:sha)
   end
 
   def diff_refs
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index c8ad7ab3e7f..a0e3c26e542 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -65,11 +65,11 @@ describe MergeRequest, models: true do
   end
 
   describe '#target_branch_sha' do
-    context 'when the target branch does not exist anymore' do
-      let(:project) { create(:project) }
+    let(:project) { create(:project) }
 
-      subject { create(:merge_request, source_project: project, target_project: project) }
+    subject { create(:merge_request, source_project: project, target_project: project) }
 
+    context 'when the target branch does not exist' do
       before do
         project.repository.raw_repository.delete_branch(subject.target_branch)
       end
@@ -78,6 +78,12 @@ describe MergeRequest, models: true do
         expect(subject.target_branch_sha).to be_nil
       end
     end
+
+    it 'returns memoized value' do
+      subject.target_branch_sha = '8ffb3c15a5475e59ae909384297fede4badcb4c7'
+
+      expect(subject.target_branch_sha).to eq '8ffb3c15a5475e59ae909384297fede4badcb4c7'
+    end
   end
 
   describe '#source_branch_sha' do
@@ -103,6 +109,12 @@ describe MergeRequest, models: true do
         expect(subject.source_branch_sha).to be_nil
       end
     end
+
+    it 'returns memoized value' do
+      subject.source_branch_sha = '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b'
+
+      expect(subject.source_branch_sha).to eq '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b'
+    end
   end
 
   describe '#to_reference' do