Fix MergeRequest#source_sha when there is no diff

`MergeRequest#source_sha` is expected to return the sha of the source
branch last commit.

But when a open Merge Request has no diff (e.g. all commits have already
been merged to the target branch), `merge_request.source_sha`
incorrectly returns `nil`.

This was un-noticed before – but now that !2217 has been merged,
it makes `Gitlab::Git::Commit.between` raise an
"Unexpected nil argument" exception.

This fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available.

1 files changed, 5 insertions, 0 deletions

diff --git a/spec/factories/merge_requests.rb b/spec/factories/merge_requests.rb
index a9df5fa1d3a..e281e2f227b 100644
--- a/spec/factories/merge_requests.rb
+++ b/spec/factories/merge_requests.rb
@@ -51,6 +51,11 @@ FactoryGirl.define do
     trait :with_diffs do
     end
 
+    trait :without_diffs do
+      source_branch "improve/awesome"
+      target_branch "master"
+    end
+
     trait :conflict do
       source_branch "feature_conflict"
       target_branch "feature"