Keep warning about unversioned trees until removed

Restore previous behavior and keep emitting a warning when an existing
gclient entry is not longer part of the client. Additionally, suggest
the usage of `gclient sync -D` to automatically remove them.

Change-Id: I62d4662ae4d0886d340230019419b68debffc5ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1483031
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>

gclient.py

@@ -1675,9 +1675,23 @@ it or fix the checkout.
             (modified_files and not self._options.force)):
           # There are modified files in this entry. Keep warning until
           # removed.
-          print(('\nWARNING: \'%s\' is no longer part of this client.  '
-                 'It is recommended that you manually remove it.\n') %
-                    entry_fixed)
+          self.add_dependency(
+              GitDependency(
+                  parent=self,
+                  name=entry,
+                  url=prev_url,
+                  managed=False,
+                  custom_deps={},
+                  custom_vars={},
+                  custom_hooks=[],
+                  deps_file=None,
+                  should_process=True,
+                  should_recurse=False,
+                  relative=None,
+                  condition=None))
+          print(('\nWARNING: \'%s\' is no longer part of this client.\n'
+                 'It is recommended that you manually remove it or use '
+                 '\'gclient sync -D\' next time.') % entry_fixed)
         else:
           # Delete the entry
           print('\n________ deleting \'%s\' in \'%s\'' % (

gclient_scm.py

@@ -222,7 +222,8 @@ class GitWrapper(SCMWrapper):
 
   def __init__(self, url=None, *args, **kwargs):
     """Removes 'git+' fake prefix from git URL."""
-    if url.startswith('git+http://') or url.startswith('git+https://'):
+    if url and (url.startswith('git+http://') or
+                url.startswith('git+https://')):
       url = url[4:]
     SCMWrapper.__init__(self, url, *args, **kwargs)
     filter_kwargs = { 'time_throttle': 1, 'out_fh': self.out_fh }