Handle `%forgesetup` in `%prep`

[sentry]

Probably it could be turned into %forgeautosetup, so that we have the git repository created. See /usr/lib/rpm/macros.d/macros.forge for the macro definition.

Without this spec-files using %forgesetup will not produce a Git repo in BUILD and conversion/update will fail.

Sentry Issue: DIST2SRC-UPDATE-B

[TomasTomecek]

Wow, interesting. I'm assuming the macro can produce a git repo so hopefully, this shouldn't be that hard to fix.