Fix multi-solution use case with conflicting custom_deps overrides.

gclient.py

Index: gclient.py
diff --git a/gclient.py b/gclient.py
index c23b0023cfc56b21c65f152a529671c65c2e4ccb..cef9f1afcf4c8659a7595a4aa6fb69c98db70bfb 100644
--- a/gclient.py
+++ b/gclient.py
@@ -325,10 +325,20 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
     # This require a full tree traversal with locks.
     siblings = [d for d in self.root.subtree(False) if d.name == self.name]
     for sibling in siblings:
-      if self.url != sibling.url:
+      self_url = self.LateOverride(self.url)
+      sibling_url = sibling.LateOverride(sibling.url)
+      # Allow to have only one to be None or ''.
+      if self_url != sibling_url and bool(self_url) == bool(sibling_url):
         raise gclient_utils.Error(
-            'Dependency %s specified more than once:\n  %s\nvs\n  %s' %
-            (self.name, sibling.hierarchy(), self.hierarchy()))
+            ('Dependency %s specified more than once:\n'
+            '  %s [%s]\n'
+            'vs\n'
+            '  %s [%s]') % (
+              self.name,
+              sibling.hierarchy(),
+              sibling_url,
+              self.hierarchy(),
+              self_url))
       # In theory we could keep it as a shadow of the other one. In
       # practice, simply ignore it.
       logging.warn('Won\'t process duplicate dependency %s' % sibling)