Improve git ignore handling

Do not add the same thing over and over again to reduce output on the CLI.

mentioned in commit composer/library/composer-json-utils@e805a3e4

mentioned in commit 7187b05b

mentioned in commit behat4drupal@8c832412

mentioned in commit ahoy@628ef50c

mentioned in commit composer/library/composer-json-utils@7f9a8386

mentioned in commit composer/library/composer-json-utils@6857146a

mentioned in commit 0adf92ee

closed