add changelog entry
replace local variables `merge_request` with instance ones
modify `MergeRequests::BuildService` to fix failed tests
extract `assign_target_project` method
remove unnecessary instance variables
remove exclamation marks and rewrite conditionals to improve readability
extract `params_does_not_contain_branches?` method, rename `unprepared_merge_request` method to `uncreatable_merge_request`
replace instance variables `merge_request` and `error_messages` with getters and setters; divide `set_title_and_description` method on two separate ones
refactor `execute` method
return `set_title_and_description` method
rename `branches_selected?` method to `branches_present?` to make it more different from `branches_specified?`
fixes after discussion
renamed method branches_valid?
fix space
return assigning methods into `execute` method
simplify `find_target_branch` and `find_source_project` methods
fix spec `merge request issuable record that supports slash commands in its description and notes`