merge: introduce mergeresult.addfile() and use it
We want to use mergeresult object at more and more places instead of this
actions dict to simplify code and further add new APIs to mergeresult object.
This patch introduces `addfile()` which adds a new file to the internal actions
dict for now.
Differential Revision: https://phab.mercurial-scm.org/D8820
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