Introduce Repository#with_repo_branch_commit

We merge repository checks inside it so we don't have to
check it on the call site, and we could also load the commit
for the caller. This greatly reduce code duplication.

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7237#note_20572919

app/models/repository.rb

@@ -1063,7 +1063,11 @@ class Repository
     Gitlab::Popen.popen(args, path_to_repo).first.lines.map(&:strip)
   end
 
-  def with_tmp_ref(source_repository, source_branch_name)
+  def with_repo_branch_commit(source_repository, source_branch_name)
+    branch_name_or_sha =
+      if source_repository == self
+        source_branch_name
+      else
         tmp_ref = "refs/tmp/#{SecureRandom.hex}/head"
 
         fetch_ref(
@@ -1072,10 +1076,13 @@ class Repository
           tmp_ref
         )
 
-    yield
+        source_repository.commit(source_branch_name).sha
+      end
+
+    yield(commit(branch_name_or_sha))
 
   ensure
-    rugged.references.delete(tmp_ref)
+    rugged.references.delete(tmp_ref) if tmp_ref
   end
 
   def fetch_ref(source_path, source_ref, target_ref)

app/services/compare_service.rb

@@ -11,19 +11,13 @@ class CompareService
   end
 
   def execute(target_project, target_branch, straight: false)
-    source_sha = source_project.repository.
-      commit(source_branch_name).try(:sha)
-
-    return unless source_sha
-
     # If compare with other project we need to fetch ref first
-    if target_project == source_project
-      compare(source_sha, target_project, target_branch, straight)
-    else
-      target_project.repository.with_tmp_ref(
-        source_project.repository, source_branch_name) do
-        compare(source_sha, target_project, target_branch, straight)
-      end
+    target_project.repository.with_repo_branch_commit(
+      source_project.repository,
+      source_branch_name) do |commit|
+      break unless commit
+
+      compare(commit.sha, target_project, target_branch, straight)
     end
   end
 

app/services/git_operation_service.rb

@@ -43,23 +43,17 @@ class GitOperationService
   def with_branch(
     branch_name,
     source_branch_name: nil,
-    source_project: repository.project)
+    source_project: repository.project,
+    &block)
 
     check_with_branch_arguments!(
       branch_name, source_branch_name, source_project)
 
-    source_commit = source_project.repository.find_branch(
-      source_branch_name || branch_name).try(:dereferenced_target)
-
     update_branch_with_hooks(branch_name) do
-      if repository.project == source_project
-        yield(source_commit)
-      else
-        repository.with_tmp_ref(
-          source_project.repository, source_branch_name) do
-            yield(source_commit)
-          end
-      end
+      repository.with_repo_branch_commit(
+        source_project.repository,
+        source_branch_name || branch_name,
+        &block)
     end
   end