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 changeset.txt
adding mymap
0 97e5f848f0936960273bbf75be6388cd0350a32b test