dispatch: fix typo suggestion for disabled extension

If the matching command lives in an in-tree extension (which is all we
scan for), and the user has disabled that extension with
"extensions.<name>=!", we were not finding it, because the path in
_disabledextensions was the empty string. If the user had set
"extensions.<name>=!<valid path>" it would work, so it seems like just
a mistake that it didn't work.

mercurial/extensions.py

@@ -494,7 +494,11 b' def _disabledpaths(strip_init=False):'
         if name in exts or name in _order or name == '__init__':
             continue
         exts[name] = path
-    exts.update(_disabledextensions)
+    for name, path in _disabledextensions.iteritems():
+        # If no path was provided for a disabled extension (e.g. "color=!"),
+        # don't replace the path we already found by the scan above.
+        if path:
+            exts[name] = path
     return exts
 
 def _moduledoc(file):

tests/test-help.t

@@ -680,7 +680,11 b' Not enabled extension gets suggested'
 Disabled extension gets suggested
   $ hg --config extensions.rebase=! rebase
   hg: unknown command 'rebase'
-  (did you mean one of rename, resolve?)
+  'rebase' is provided by the following extension:
+  
+      rebase        command to move sets of revisions to a different ancestor
+  
+  (use 'hg help extensions' for information on enabling extensions)
   [255]
 
 Make sure that we don't run afoul of the help system thinking that