children: don't pass filectx to displayer

displayer doesn't want a fctx but a ctx. It failed with -Tdefault template.

    Traceback (most recent call last):
      ...
      File "mercurial/templatekw.py", line 212, in showbookmarks
        bookmarks = args['ctx'].bookmarks()
    AttributeError: 'filectx' object has no attribute 'bookmarks'

hgext/children.py

@@ -39,11 +39,13 b' def children(ui, repo, file_=None, **opt'
     """
     rev = opts.get('rev')
     if file_:
-        ctx = repo.filectx(file_, changeid=rev)
+        fctx = repo.filectx(file_, changeid=rev)
+        childctxs = [fcctx.changectx() for fcctx in fctx.children()]
     else:
         ctx = repo[rev]
+        childctxs = ctx.children()
 
     displayer = cmdutil.show_changeset(ui, repo, opts)
-    for cctx in ctx.children():
+    for cctx in childctxs:
         displayer.show(cctx)
     displayer.close()

tests/test-children.t

@@ -122,4 +122,12 b' hg children file0 at revision 0 (should '
   summary:     2
   
 
+should be compatible with templater (don't pass fctx to displayer)
+  $ hg children file0 -Tdefault
+  changeset:   2:8f5eea5023c2
+  user:        test
+  date:        Thu Jan 01 00:00:02 1970 +0000
+  summary:     2
+  
+
   $ cd ..