Add option for git-repo to support 'silent' uploads

When --ne/--no-emails is added to 'repo upload' command line, gerrit
server will not generate notification emails.

project.py:Project.UploadForReview method is modified to accept a
string recognizable by gerrit to indicate different sets of destination
email addressees, but the upload command line allows only one option -
disable sending emails completely.

Default repo upload behavior is not being changed.

TEST=tried in the Chrome OS repo, observed that patches uploaded with
     --ne or --no-emails indeed do not trigger any emails, while
     patches uploaded without these command line options still trigger
     email notifications.

Change-Id: I0301edec984907aedac277d883bd0e6d3099aedc
diff --git a/subcmds/upload.py b/subcmds/upload.py
index 02b43b4..acb9d7f 100644
--- a/subcmds/upload.py
+++ b/subcmds/upload.py
@@ -150,6 +150,9 @@
     p.add_option('-d', '--draft',
                  action='store_true', dest='draft', default=False,
                  help='If specified, upload as a draft.')
+    p.add_option('--ne', '--no-emails',
+                 action='store_false', dest='notify', default=True,
+                 help='If specified, do not send emails on upload.')
     p.add_option('-p', '--private',
                  action='store_true', dest='private', default=False,
                  help='If specified, upload as a private change.')
@@ -391,6 +394,7 @@
                                auto_topic=opt.auto_topic,
                                draft=opt.draft,
                                private=opt.private,
+                               notify=None if opt.notify else 'NONE',
                                wip=opt.wip,
                                dest_branch=destination,
                                validate_certs=opt.validate_certs,