Warn before uploading more than one commit to Gerrit.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index e8cadf270d1d9a7f85aed299e41f2bb328499b66..f99d0913bb1cd8cc6cde201adfa85e3b39a40e89 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1494,6 +1494,16 @@ def GerritUpload(options, args, cl):
     return 1
   if CHANGE_ID not in change_desc.description:
     AddChangeIdToCommitMessage(options, args)
+
+  commits = RunGit(['rev-list', '%s/%s...' % (remote, branch)]).splitlines()
+  if len(commits) > 1:
+    print('WARNING: This will upload %d commits. Run the following command '
+          'to see which commits will be uploaded: ' % len(commits))
+    print('git log %s/%s...' % (remote, branch))
+    print('You can also use `git squash-branch` to squash these into a single'
+          'commit.')
+    ask_for_data('About to upload; enter to confirm.')
+
   if options.reviewers:
     change_desc.update_reviewers(options.reviewers)