API support for squash commit message during merge

Issues
https://gitlab.com/gitlab-org/gitlab-ce/issues/47149
https://gitlab.com/gitlab-org/gitlab-ce/issues/56014

changelogs/unreleased/56014-api-merge-request-squash-commit-messages.yml

+---
+title: API allows setting the squash commit message when squashing a merge request
+merge_request: 24784
+author:
+type: added

doc/api/merge_requests.md

@@ -994,6 +994,8 @@ Parameters:
 - `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
 - `merge_request_iid` (required)            - Internal ID of MR
 - `merge_commit_message` (optional)         - Custom merge commit message
+- `squash_commit_message` (optional)        - Custom squash commit message
+- `squash` (optional)                       - if `true` the commits will be squashed into a single commit on merge
 - `should_remove_source_branch` (optional)  - if `true` removes the source branch
 - `merge_when_pipeline_succeeds` (optional) - if `true` the MR is merged when the pipeline succeeds
 - `sha` (optional)                          - if present, then this SHA must match the HEAD of the source branch, otherwise the merge will fail

lib/api/merge_requests.rb

@@ -343,6 +343,7 @@ module API
       end
       params do
         optional :merge_commit_message, type: String, desc: 'Custom merge commit message'
+        optional :squash_commit_message, type: String, desc: 'Custom squash commit message'
         optional :should_remove_source_branch, type: Boolean,
                                                desc: 'When true, the source branch will be deleted if possible'
         optional :merge_when_pipeline_succeeds, type: Boolean,
@@ -370,6 +371,7 @@ module API
 
         merge_params = {
           commit_message: params[:merge_commit_message],
+          squash_commit_message: params[:squash_commit_message],
           should_remove_source_branch: params[:should_remove_source_branch]
         }
 

spec/requests/api/merge_requests_spec.rb

@@ -951,6 +951,29 @@ describe API::MergeRequests do
 
       expect(response).to have_gitlab_http_status(404)
     end
+
+    describe "the squash_commit_message param" do
+      let(:squash_commit) do
+        project.repository.commits_between(json_response['diff_refs']['start_sha'], json_response['merge_commit_sha']).first
+      end
+
+      it "results in a specific squash commit message when set" do
+        squash_commit_message = 'My custom squash commit message'
+
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user), params: {
+          squash: true,
+          squash_commit_message: squash_commit_message
+        }
+
+        expect(squash_commit.message.chomp).to eq(squash_commit_message)
+      end
+
+      it "results in a default squash commit message when not set" do
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user), params: { squash: true }
+
+        expect(squash_commit.message).to eq(merge_request.default_squash_commit_message)
+      end
+    end
   end
 
   describe "PUT /projects/:id/merge_requests/:merge_request_iid" do