Refactor MergeRequestDiff model

Since MergeRequestDiff is not about branches and current state of merge
request diff anymore I removed most of branch related method and added
validation for head/start/base commit sha. From this point
MergeRequestDiff is about saving diff between branches only once at moment of
creation. Once created MergeRequestDiff should not be changes. Because
of that we should not rely on changes in source/target branches when
read from MergeRequestDiff
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request_diff.rb

@@ -8,9 +8,6 @@ class MergeRequestDiff < ActiveRecord::Base
 
   belongs_to :merge_request
 
-  delegate :source_branch_sha, :target_branch_sha,
-    :target_branch, :source_branch, to: :merge_request, prefix: nil
-
   state_machine :state, initial: :empty do
     state :collected
     state :overflow
@@ -25,14 +22,23 @@ class MergeRequestDiff < ActiveRecord::Base
   serialize :st_commits
   serialize :st_diffs
 
-  after_initialize :set_diff_range
-  after_create :reload_content, unless: :importing?
+  validates :head_commit_sha,  presence: true
+  validates :start_commit_sha, presence: true
+  validates :base_commit_sha,  presence: true
+
+  after_initialize :ensure_head_commit_sha, if: :persisted?
+  before_create :set_diff_range,   unless: :importing?
+  after_create :reload_content,    unless: :importing?
   after_save :keep_around_commits, unless: :importing?
 
+  def ensure_head_commit_sha
+    self.head_commit_sha ||= last_commit.sha
+  end
+
   def set_diff_range
-    self.start_commit_sha ||= target_branch_sha
-    self.head_commit_sha  ||= source_branch_sha
-    self.base_commit_sha  ||= branch_base_sha
+    self.start_commit_sha ||= merge_request.target_branch_sha
+    self.head_commit_sha  ||= merge_request.source_branch_sha
+    self.base_commit_sha  ||= find_base_sha
   end
 
   def reload_content
@@ -199,14 +205,10 @@ class MergeRequestDiff < ActiveRecord::Base
     project.repository
   end
 
-  def branch_base_commit
-    return unless source_branch_sha && target_branch_sha
-
-    project.merge_base_commit(source_branch_sha, target_branch_sha)
-  end
+  def find_base_sha
+    return unless head_commit_sha && start_commit_sha
 
-  def branch_base_sha
-    branch_base_commit.try(:sha)
+    project.merge_base_commit(head_commit_sha, start_commit_sha).try(:sha)
   end
 
   def utf8_st_diffs