purge: also deal with directory with --confirm

Getting the exact number is simple (you have to project the result of previous
deletion to detect super-directory with only about-to-be-deleted content). So we
use a vaguer message.

Differential Revision: https://phab.mercurial-scm.org/D9819

mercurial/merge.py

@@ -2386,6 +2386,16 b' def purge('
             elif nb_ignored:
                 msg = _(b"permanently delete %d ignored files?")
                 msg %= nb_ignored
+            elif removeemptydirs:
+                dir_count = 0
+                for f in directories:
+                    if matcher(f) and not repo.wvfs.listdir(f):
+                        dir_count += 1
+                if dir_count:
+                    msg = _(
+                        b"permanently delete at least %d empty directories?"
+                    )
+                    msg %= dir_count
             else:
                 # XXX we might be missing directory there
                 return res

tests/test-purge.t

@@ -23,6 +23,10 b' delete an empty directory'
   $ mkdir empty_dir
   $ hg purge -p -v
   empty_dir
+  $ hg purge --confirm
+  permanently delete at least 1 empty directories? (yN) n
+  abort: removal cancelled
+  [250]
   $ hg purge -v
   removing directory empty_dir
   $ ls -A