Set default squash message to MR title

The last commit is a bad default to use, because if you want to squash
commits, the last commit message will often be of the type 'fix
specs'. Instead, use the MR title and author as the commit's author.

app/services/merge_requests/squash_service.rb

@@ -40,9 +40,12 @@ module MergeRequests
       )
 
       run_git_command(
-        %W(commit -C #{merge_request.diff_head_sha}),
+        %W(commit -m #{merge_request.title}),
         tree_path,
-        git_env.merge('GIT_COMMITTER_NAME' => current_user.name, 'GIT_COMMITTER_EMAIL' => current_user.email),
+        git_env.merge('GIT_COMMITTER_NAME' => current_user.name,
+                      'GIT_COMMITTER_EMAIL' => current_user.email,
+                      'GIT_AUTHOR_NAME' => merge_request.author.name,
+                      'GIT_AUTHOR_EMAIL' => merge_request.author.email),
         'commit squashed changes'
       )
 

doc/user/project/merge_requests/squash_and_merge.md

@@ -35,8 +35,8 @@ This can then be overridden at the time of accepting the merge request:
 
 The squashed commit has the following metadata:
 
-* Message: taken from the last commit in the source branch.
-* Author: taken from the last commit in the source branch.
+* Message: the title of the merge request.
+* Author: the author of the merge request.
 * Committer: the user who initiated the squash.
 
 ## Squashing and [fast-forward merge][ff-merge]

spec/features/merge_requests/squash_spec.rb

@@ -12,11 +12,10 @@ feature 'Squashing merge requests', js: true, feature: true do
   shared_examples 'squash' do
     it 'squashes the commits into a single commit, and adds a merge commit' do
       latest_master_commits = project.repository.commits_between(original_head.sha, 'master').map(&:raw)
-      last_mr_commit = project.repository.commit(source_branch)
 
       squash_commit = an_object_having_attributes(sha: a_string_matching(/\h{40}/),
-                                                  message: "#{last_mr_commit.message}\n",
-                                                  author_name: last_mr_commit.author_name,
+                                                  message: "Csv\n",
+                                                  author_name: user.name,
                                                   committer_name: user.name)
 
       merge_commit = an_object_having_attributes(sha: a_string_matching(/\h{40}/),

spec/services/merge_requests/squash_service_spec.rb

@@ -54,14 +54,12 @@ describe MergeRequests::SquashService do
         let(:squash_sha) { service.execute(merge_request)[:squash_sha] }
         let(:squash_commit) { project.repository.commit(squash_sha) }
 
-        it 'copies the author info and message from the last commit in the source branch' do
-          diff_head_commit = merge_request.diff_head_commit
+        it 'copies the author info and message from the merge request' do
+          expect(squash_commit.author_name).to eq(merge_request.author.name)
+          expect(squash_commit.author_email).to eq(merge_request.author.email)
 
-          expect(squash_commit.author_name).to eq(diff_head_commit.author_name)
-          expect(squash_commit.author_email).to eq(diff_head_commit.author_email)
-
-          # The commit message on the 'real' commit doesn't have a trailing newline
-          expect(squash_commit.message.chomp).to eq(diff_head_commit.message)
+          # Commit messages have a trailing newline, but titles don't.
+          expect(squash_commit.message.chomp).to eq(merge_request.title)
         end
 
         it 'sets the current user as the committer' do