git cl: use project~number on Gerrit for better routing when setting reviewers

R=ehmaldonado@chromium.org

Testing
  patched my own depot_tools in $PATH and uploaded
  https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1186072
  which set reviewers and ccs as expected.

Bug: 876910
Change-Id: I43c0f2284941cf703133bb51132226d4a0472d8e
Reviewed-on: https://chromium-review.googlesource.com/1186068
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/gerrit_util.py b/gerrit_util.py
index d5780ad..610d9b3 100644
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -900,3 +900,14 @@
   finally:
     if tdir:
       gclient_utils.rmtree(tdir)
+
+
+def ChangeIdentifier(project, change_number):
+  """Returns change identifier "project~number" suitable for |chagne| arg of
+  this module API.
+
+  Such format is allows for more efficient Gerrit routing of HTTP requests,
+  comparing to specifying just change_number.
+  """
+  assert int(change_number)
+  return '%s~%s' % (urllib.quote(project, safe=''), change_number)