Fix finding project by identifier

I was trying to build zanshin with translations in Arch Linux, and figured out that currently significant crutches are required to do it. My current wip pkgbuild is here. This mr fixes a problem of makepkg creating makepkg branch and has no origin, at which scripts relied. First I wanted to create a temporary script just to reliably extract repopath. But as I can see, fetchpo is only used by cmake ecm, so I edited it directly.