hg merge with uncommited ends up with them committed togethe

C:\***\test>hg init

C:\***\test>copy con f
1
2
3
^Z
        1 file(s) copied.

C:\***\test>hg add f

C:\***\test>hg commit -m init

C:\***\test>copy con f
1
Overwrite f? (Yes/No/All): y
2a
3
^Z
        1 file(s) copied.

C:\***\test>hg commit -m a

C:\***\test>hg update 0
0 files updated, 0 files merged, 0 files removed, 0 files unresolved

C:\***\test>copy con f
1
Overwrite f? (Yes/No/All): y
2b
3
^Z
        1 file(s) copied.

C:\***\test>hg commit -m b
created new head

C:\***\test>copy con f
1
Overwrite f? (Yes/No/All): y
2bc
3
^Z
        1 file(s) copied.

C:\***\test>hg merge 1
abort: uncommitted changes
(use 'hg status' to list changes)

C:\***\test>hg merge --force 1
merging f
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)

C:\***\test>hg commit
abort: empty commit message

C:\***\test>hg commit -m merge

C:\***\test>hg status f

C:\***\test>hg diff -r 2
diff -r 81867d6c4ada f
--- a/f Fri Oct 21 08:39:13 2016 +0300
+++ b/f Fri Oct 21 08:45:29 2016 +0300
@@ -1,3 +1,3 @@
 1
-2b
+2bc
 3