Use git worktree in git_build_app_repo() for git >= 2.11
Use git worktree in git_build_app_repo() for git >= 2.11

Git 2.11.0 breaks the git clone approach in git_build_app_repo() in
https://github.com/git/git/commit/25ab004c53cdcfea485e5bf437aeaa74df47196d

git worktree is a much simpler approach but only works for git >= 2.5
so it is not available in Ubuntu 14.04.

Fixes #2512.

progrium/dokku,progrium/dokku,elia/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,jimjag/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,alexquick/dokku,progrium/dokku,jimjag/dokku,alexquick/dokku,jimjag/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,fruitl00p/dokku,elia/dokku,alexquick/dokku,jimjag/dokku,progrium/dokku,elia/dokku,elia/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,progrium/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,fruitl00p/dokku,progrium/dokku,alexquick/dokku,jimjag/dokku,alexquick/dokku,elia/dokku,claytonbrown/dokku,dokku/dokku,alexquick/dokku,alexquick/dokku,fruitl00p/dokku,elia/dokku,jimjag/dokku,alexquick/dokku,elia/dokku,claytonbrown/dokku,elia/dokku,fruitl00p/dokku,fruitl00p/dokku,fruitl00p/dokku,dokku/dokku,fruitl00p/dokku,fruitl00p/dokku