changectx: extract explicit computechangesetfilesremoved method from context

Right now, the logic around changeset centric removed files data are buried into
the "changectx" code. We extract this code in a dedicated method (in the scmutil
module) for clarity. This clarity will help to explicitly compute and caches
these data in the future.

mercurial/context.py

@@ -466,12 +466,7 b' class changectx(basectx):'
             (source == 'compatibility' and
              self._changeset.filesremoved is not None)):
             return self._changeset.filesremoved or []
-
-        removed = []
-        for f in self.files():
-            if f not in self:
-                removed.append(f)
-        return removed
+        return scmutil.computechangesetfilesremoved(self)
 
     @propertycache
     def _copies(self):

mercurial/scmutil.py

@@ -1993,3 +1993,12 b' def computechangesetfilesadded(ctx):'
         if not any(f in p for p in ctx.parents()):
             added.append(f)
     return added
+
+def computechangesetfilesremoved(ctx):
+    """return the list of files removed in a changeset
+    """
+    removed = []
+    for f in ctx.files():
+        if f not in ctx:
+            removed.append(f)
+    return removed