repo download: add --revert option

BZ: 4779
Allows to revert a gerrit patch
This patch is necessary for the on-demand creation of
engineering builds using buildbot

You can now use:
repo download [--revert|-r project changeid/patchnumber

This is useful to automate reverting of a patch
in the context of build automation, and regression bisection

Change-Id: I3985e80e4b2a230f83526191ea1379765a54bdcf
Signed-off-by: Erwan Mahe <erwan.mahe@intel.com>
Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
diff --git a/subcmds/download.py b/subcmds/download.py
index 79d0192..f79f485 100644
--- a/subcmds/download.py
+++ b/subcmds/download.py
@@ -36,6 +36,9 @@
     p.add_option('-c','--cherry-pick',
                  dest='cherrypick', action='store_true',
                  help="cherry-pick instead of checkout")
+    p.add_option('-r','--revert',
+                 dest='revert', action='store_true',
+                 help="revert instead of checkout")
 
   def _ParseChangeIds(self, args):
     if not args:
@@ -68,7 +71,7 @@
           % (project.name, change_id, ps_id)
         sys.exit(1)
 
-      if not dl.commits:
+      if not opt.revert and not dl.commits:
         print >>sys.stderr, \
           '[%s] change %d/%d has already been merged' \
           % (project.name, change_id, ps_id)
@@ -82,5 +85,7 @@
           print >>sys.stderr, '  %s' % (c)
       if opt.cherrypick:
         project._CherryPick(dl.commit)
+      elif opt.revert:
+        project._Revert(dl.commit)
       else:
         project._Checkout(dl.commit)