revert: detect unknown files in the same path as files marked as removed

Such unknown files may need to be backed up. Having them identified beforehand
will help simplify the backup logic.

We now use different sets with different backup strategies.

mercurial/cmdutil.py

@@ -2492,6 +2492,24 b' def revert(ui, repo, ctx, parents, *pats'
                 dsremoved.add(src)
                 names[src] = (repo.pathto(src, cwd), True)
 
+        # For files marked as removed, we check if an unknown file is present at
+        # the same path. If a such file exists it may need to be backed up.
+        # Making the distinction at this stage helps have simpler backup
+        # logic.
+        removunk = set()
+        for abs in removed:
+            target = repo.wjoin(abs)
+            if os.path.lexists(target):
+                removunk.add(abs)
+        removed -= removunk
+
+        dsremovunk = set()
+        for abs in dsremoved:
+            target = repo.wjoin(abs)
+            if os.path.lexists(target):
+                dsremovunk.add(abs)
+        dsremoved -= dsremovunk
+
         ## computation of the action to performs on `names` content.
 
         def removeforget(abs):
@@ -2528,9 +2546,13 b' def revert(ui, repo, ctx, parents, *pats'
             # Added since target
             (dsadded,       actions['remove'],   discard),
             # Removed since  target, before working copy parent
-            (removed,       actions['add'],      backup),
+            (removed,       actions['add'],      discard),
+            # Same as `removed` but an unknown file exists at the same path
+            (removunk,      actions['add'],      backup),
             # Removed since targe, marked as such in working copy parent
-            (dsremoved,     actions['undelete'], backup),
+            (dsremoved,     actions['undelete'], discard),
+            # Same as `dsremoved` but an unknown file exists at the same path
+            (dsremovunk,    actions['undelete'], backup),
             ## the following sets does not result in any file changes
             # File with no modification
             (clean,         actions['noop'],     discard),