Add ability to overwrite a branch via Commits API

For convenience, rather than deleting a branch and recreating the
branch, the `force` option emulates a Git force push overwriting the
existing branch.

app/services/commits/create_service.rb

@@ -11,6 +11,7 @@ module Commits
       @start_project = params[:start_project] || @project
       @start_branch = params[:start_branch]
       @branch_name = params[:branch_name]
+      @force = params[:force] || false
     end
     def execute
@@ -60,13 +61,13 @@ module Commits
     end
     def validate_branch_existance!
-      if !project.empty_repo? && different_branch? && repository.branch_exists?(@branch_name)
+      if !project.empty_repo? && different_branch? && repository.branch_exists?(@branch_name) && @force == false
         raise_error("A branch called '#{@branch_name}' already exists. Switch to that branch in order to make changes")
       end
     end
     def validate_new_branch_name!
-      result = ValidateNewBranchService.new(project, current_user).execute(@branch_name)
+      result = ValidateNewBranchService.new(project, current_user).execute(@branch_name, @force)
       if result[:status] == :error
         raise_error("Something went wrong when we tried to create '#{@branch_name}' for you: #{result[:message]}")

app/services/validate_new_branch_service.rb

@@ -3,14 +3,14 @@
 require_relative 'base_service'
 class ValidateNewBranchService < BaseService
-  def execute(branch_name)
+  def execute(branch_name, force)
     valid_branch = Gitlab::GitRefValidator.validate(branch_name)
     unless valid_branch
       return error('Branch name is invalid')
     end
-    if project.repository.branch_exists?(branch_name)
+    if project.repository.branch_exists?(branch_name) && force == false
       return error('Branch already exists')
     end

lib/api/commits.rb

@@ -98,6 +98,7 @@ module API
         optional :start_branch, type: String, desc: 'Name of the branch to start the new commit from'
         optional :author_email, type: String, desc: 'Author email for commit'
         optional :author_name, type: String, desc: 'Author name for commit'
+        optional :force, type: Boolean, default: false, desc: 'When `true` overwrite the target branch with a new branch starting from the `start_branch`'
       end
       post ':id/repository/commits' do
         authorize_push_to_branch!(params[:branch])