merge: use manifestdict.walk() instead of manifestdict.matches()

As with other patches in this series, this avoids making a
potentially-expensive copy of a manifest.

Differential Revision: https://phab.mercurial-scm.org/D8084

mercurial/merge.py

@@ -997,11 +997,10 b' def _checkcollision(repo, wmf, actions):'
     """
     Check for case-folding collisions.
     """
-
     # If the repo is narrowed, filter out files outside the narrowspec.
     narrowmatch = repo.narrowmatch()
     if not narrowmatch.always():
-        wmf = wmf.matches(narrowmatch)
+        pmmf = set(wmf.walk(narrowmatch))
         if actions:
             narrowactions = {}
             for m, actionsfortype in pycompat.iteritems(actions):
@@ -1010,9 +1009,9 b' def _checkcollision(repo, wmf, actions):'
                     if narrowmatch(f):
                         narrowactions[m].append((f, args, msg))
             actions = narrowactions
-
-    # build provisional merged manifest up
-    pmmf = set(wmf)
+    else:
+        # build provisional merged manifest up
+        pmmf = set(wmf)
 
     if actions:
         # KEEP and EXEC are no-op