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.
EVERYTHING
diff -r ec612a6291f1 alpha/one
--- a/alpha/one
+++ b/alpha/one
@@ -0,0 +1,1 @@
+1
diff -r ec612a6291f1 beta/two
--- a/beta/two
+++ b/beta/two
@@ -0,0 +1,1 @@
+2
BETA ONLY
diff -r ec612a6291f1 beta/two
--- a/beta/two
+++ b/beta/two
@@ -0,0 +1,1 @@
+2
INSIDE BETA
diff -r ec612a6291f1 beta/two
--- a/beta/two
+++ b/beta/two
@@ -0,0 +1,1 @@
+2