tests/test-merge-default

imerge: Fix unbound name error and add a test case

#!/bin/sh

hg init
echo a > a
hg commit -A -ma

echo a >> a
hg commit -mb

echo a >> a
hg commit -mc

hg up 1
echo a >> a
hg commit -md

hg up 1
echo a >> a
hg commit -me

hg up 1
echo % should fail because not at a head
hg merge

hg up
echo % should fail because \> 2 heads
hg merge

echo % should succeed
hg merge 2
hg commit -mm1

echo % should succeed - 2 heads
hg merge
hg commit -mm2

echo % should fail because at tip
hg merge

hg up 0
echo % should fail because 1 head
hg merge

true