purge: prevent a silly crash with --confirm --files

if --files is passed, there was no directory to checks and `msg` was undefined.
This is now fixed and tested.

mercurial/merge.py

@@ -2442,6 +2442,7 b' def purge('
         status = repo.status(match=matcher, ignored=ignored, unknown=unknown)
 
         if confirm:
+            msg = None
             nb_ignored = len(status.ignored)
             nb_unknown = len(status.unknown)
             if nb_unknown and nb_ignored:
@@ -2463,12 +2464,12 b' def purge('
                         b"permanently delete at least %d empty directories?"
                     )
                     msg %= dir_count
-                else:
-                    # XXX we might be missing directory there
-                    return res
-            msg += b" (yN)$$ &Yes $$ &No"
-            if repo.ui.promptchoice(msg, default=1) == 1:
-                raise error.CanceledError(_(b'removal cancelled'))
+            if msg is None:
+                return res
+            else:
+                msg += b" (yN)$$ &Yes $$ &No"
+                if repo.ui.promptchoice(msg, default=1) == 1:
+                    raise error.CanceledError(_(b'removal cancelled'))
 
         if removefiles:
             for f in sorted(status.unknown + status.ignored):

tests/test-purge.t

@@ -350,4 +350,9 b' remove both files and dirs'
   .hg
   .hgignore
 
+Test some --confirm case that ended crashing
+
+  $ hg purge --confirm
+  $ hg purge --confirm --all --files
+
   $ cd ..