$ 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
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo "something else" > a
$ hg commit -m branch2 -d "1000000 0"
created new head
$ hg merge 1
merging a
warning: conflicts during merge.
merging a failed!
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges or 'hg update -C' to abandon
$ hg id
e7fe8eb3e180+0d24b7662d3e+ tip
$ cat a
<<<<<<< local
something else
=======
something
>>>>>>> other
$ hg status
M a
? a.orig