merge: make mergeresult constructor initialize empty object
In future patches, we will be going to update mergeresult object
instead of building an actions dict and then setting it in the object.
Differential Revision: https://phab.mercurial-scm.org/D8819
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..