Correct error message returned by ChangeService

Previously the string was spanning multiple lines and included a
needless `\n` character in the resulting error message.

This change also reduces duplication by assigning two variables.

app/services/commits/change_service.rb

@@ -24,8 +24,12 @@ module Commits
         start_project: @start_project,
         start_branch_name: @start_branch)
     rescue Gitlab::Git::Repository::CreateTreeError
-      error_msg = "Sorry, we cannot #{action.to_s.dasherize} this #{@commit.change_type_title(current_user)} automatically.
-                   This #{@commit.change_type_title(current_user)} may already have been #{action.to_s.dasherize}ed, or a more recent commit may have updated some of its content."
+      act = action.to_s.dasherize
+      type = @commit.change_type_title(current_user)
+
+      error_msg = "Sorry, we cannot #{act} this #{type} automatically. " \
+        "This #{type} may already have been #{act}ed, or a more recent " \
+        "commit may have updated some of its content."
       raise ChangeError, error_msg
     end
   end