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

mercurial/merge.py

@@ -414,12 +414,13 def checkpathconflicts(repo, wctx, mctx,
     # The set of files deleted by all the actions.
     deletedfiles = set()
 
-    for f, (m, args, msg) in mresult.actions.items():
-        if m in (
+    for (f, args, msg) in mresult.getactions(
+        (
             mergestatemod.ACTION_CREATED,
             mergestatemod.ACTION_DELETED_CHANGED,
             mergestatemod.ACTION_MERGE,
             mergestatemod.ACTION_CREATED_MERGE,
+        )
         ):
             # This action may create a new local file.
             createdfiledirs.update(pathutil.finddirs(f))
@@ -429,13 +430,15 def checkpathconflicts(repo, wctx, mctx,
                 # will be checked once we know what all the deleted files are.
                 remoteconflicts.add(f)
         # Track the names of all deleted files.
-        if m == mergestatemod.ACTION_REMOVE:
+    for (f, args, msg) in mresult.getactions((mergestatemod.ACTION_REMOVE,)):
             deletedfiles.add(f)
-        if m == mergestatemod.ACTION_MERGE:
+    for (f, args, msg) in mresult.getactions((mergestatemod.ACTION_MERGE,)):
             f1, f2, fa, move, anc = args
             if move:
                 deletedfiles.add(f1)
-        if m == mergestatemod.ACTION_DIR_RENAME_MOVE_LOCAL:
+    for (f, args, msg) in mresult.getactions(
+        (mergestatemod.ACTION_DIR_RENAME_MOVE_LOCAL,)
+    ):
             f2, flags = args
             deletedfiles.add(f2)