tests/test-cat.out

merge: when current branch has 1 or > 2 heads, actually abort.

Currently merge just prints abort-like messages to stderr and then
exits with a misleading status 0 (cleverly disguised as "False").
With this change it raises Abort, just like every other fatal error.

adding a
adding b
0
0
0
0
a: No such file in rev 03f6b0774996
1