buildserver: run dir check as very first thing
buildserver: run dir check as very first thing

No point in running any other code if the script is just going to bail
out with an error.  This assumes that ./makebuildserver is only ever
run from a git clone of fdroidserver.git.

f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver