don't overwrite git repo remote
It seems kde-builder will re-set default remote every time it updates the repo. If the existing remote url is different from the "official" url, the old url get overwritten. There are valid user cases for custom set remote urls. For example, I didn't have gitlab account and password saved in git config, I set remote url to git address on projects I'm working on. The existing remote url shouldn't be overwritten by the default value, imo.
This patch skip setting remote if it already exists.