Add a features list to branches.cache to detect caches of old hg versions. The leading space in the written file makes sure that the feature list never can match an existing version, even if the first feature can be read as hex. Additionally old hg versions display the id with --debug, too.

#!/bin/sh

hg init
echo "nothing" > a
hg add a
hg commit -m ancestor -d "1000000 0"
echo "something" > a
hg commit -m branch1 -d "1000000 0"
hg co 0
echo "something else" > a
hg commit -m branch2 -d "1000000 0"
HGMERGE=merge; export HGMERGE
hg merge 1
hg id
egrep -v ">>>|<<<" a
hg status