Since upgrading my Leap 15.0 system to Leap 15.1, "osc build" requires current
directory to be a checked out working copy even when "--alternative-project"
argument is used:

> mike@unicorn:~/work/git/sle-ksource/kernel-source> osc build --alternative-project openSUSE:Factory kernel-default.spec 
> Error: "/srv/git/sle-ksource/kernel-source" is not an osc package working copy.

Leap 15.0 and 42.3 version did not require this and the example used to work
with only package contents in current directory (and no .osc/* metadata).

Last known good version:  0.162.1-lp150.1.3  (Leap 15.0)
First known bad version:  0.165.0-lp151.1.1  (Leap 15.1)

According to Ji���� Slab��, 0.164.2 also works.