Update message when external changes are detected

Provide a link to the external change patch diff and clarify prompt.

R=gavinmak@google.com

Bug: 1441902
Change-Id: I9e4853959fbe46c1e4e887d3a0d9e727570b22d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4564233
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>

git_cl.py

@@ -2972,8 +2972,11 @@ class Changelist(object):
       desc = ps_to_info[ps].get('description', '')
       print('Patchset %d [%s] %s' % (ps, commit, desc))
 
-    if not ask_for_explicit_yes('\nUploading as-is will override them. '
-                                'Get the latest changes and apply?'):
+    print('\nSee diff at: %s/%d..%d' %
+          (self.GetIssueURL(short=True), local_ps, external_ps))
+    print('\nUploading without applying patches will override them.')
+
+    if not ask_for_explicit_yes('Get the latest changes and apply on top?'):
       return
 
     # Get latest Gerrit merge base. Use the first parent even if multiple exist.