builder: Properly check for gitlink trees
builder: Properly check for gitlink trees

Instead of expecting the second word to be "commit", we should do like
git does and check for the file mode, which is 160000 for gitlink trees.

https://github.com/git/git/blob/cda1bbd474805e653dda8a71d4ea3790e2a66cbb/cache.h#L96

handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak