Try uncommitting a commit with not exactly one parent

This should fail cleanly -- and in fact it does. Except for printing
an assertion backtrace instead of a nice error message. (This is a
regression introduced by the conversion of "stg uncommit" to the new
infrastructure.)

Found by Erik Sandberg <mandolaerik@gmail.com>.

Signed-off-by: Karl Hasselström <kha@treskal.com>