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
marked working directory as branch b
adding b
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
fast-forward
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
bogus fast-forward should fail
abort: can't merge with ancestor
done