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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.