builder: Check if .gitmodules exists before showing it. Fixes #366
builder: Check if .gitmodules exists before showing it. Fixes #366

This commit checks if <revision>:.gitmodules exists in the repo before
calling git-show on it, so an error message doesn't show up in the
program's output.

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