CONTRIBUTING

merge: use the new action based mapping introduced in mergeresult obj

Before this patch, we have good number of instances of following types:

```
for f, (m, args, msg) in mresult.actions:
    if m == ACTION_X:
        do_something
```

We iterate over the whole list and then filter for a certain action. Previous
patch introduced a action based mapping in mergeresult object. The above code
now looks like:

```
for f, args, msg in mresult.getactions([ACTION_X, ...]):
    do_something
```

Differential Revision: https://phab.mercurial-scm.org/D8831

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