don't clone gtk+ again when there is already one
@pksadiq
Submitted by Mohammed Sadiq Link to original bug (#772715)
Description
Since gtk+ is branched for gtk4, jhbuild clones the repo of gtk+ and save it to gtk+-3 directory, even when a gtk+ repo is already present.
Instead of cloning again, it might be nice to reuse the current gtk+ repo and checkout the gtk3 branch, copy to a new directory, and use it.