mergeresult: add `files()` and use it `files()` will return a list of files on which an action needs to be performed. This is a step to stop exposing the underlying map to the user of this object. Differential Revision: https://phab.mercurial-scm.org/D8887

mergeresult: introduce getfile() and use it where required We want to hide the underlying dictionary from the users and provide API for valid and sane use cases. Differential Revision: https://phab.mercurial-scm.org/D8886

merge: use ACTION_* constants instead of values in _filternarrowactions() It makes easier to check what noconflicttypes are and which are not included. I hope we can have a state where we always use ACTION_* constants instead of these values which are very hard to understand. Differential Revision: https://phab.mercurial-scm.org/D8885

merge: rework iteration over mergeresult object in checkpathconflicts() Instead of following pattern: ``` for f, (m, args, msg) in mresult.actions.items(): if m == mergestatemod.ACTION_*: ... elif m == mergestatemod.ACTION_*: ... .... ``` We do: ``` for (f, args, msg) in mresult.getaction((mergestatemod.ACTION_*,)): ... for (f, args, msg) in mresult.getaction((mergestatemod.ACTION_*,)): ... .... ``` This makes code bit easier to understand and prevent iterating over actions which we don't need. Differential Revision: https://phab.mercurial-scm.org/D8884

applyupdates: simplfy calculation of number of updated files Instead of increasing the `updated` variable each time in a loop, let's use the mergeresult object API to calculate the updated value in one call. Differential Revision: https://phab.mercurial-scm.org/D8883

mergeresult: yield from getactions() instead of buidling a list and returning Only 7 out of 29 callers change the underlying dict while iterating. So it's better to yield and wrap the 7 callers with `list()`. Differential Revision: https://phab.mercurial-scm.org/D8882