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.
#!/bin/sh
"$TESTDIR/hghave" fifo || exit 80
hg init
mkfifo p
hg serve --stdio < p &
P=$!
(echo lock; echo addchangegroup; sleep 5) > p &
Q=$!
sleep 3
kill -HUP $P
wait
echo .hg/* .hg/store/*