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
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