Handle spaces in Gerrit search options

Gerrit rejects requests with 400 Bad request if query string `q`
contains any spaces. Replace spaces with a plus sign solves the problem.

R=thakis@chromium.org

Bug: 1199692
Change-Id: Ic13dda378527594c6cf57b8cb2edf740517811ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2832653
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
diff --git a/gerrit_util.py b/gerrit_util.py
index 4d817c3..81d0f88 100644
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -85,7 +85,7 @@
   https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#list-changes
   """
   q = [urllib.parse.quote(first_param)] if first_param else []
-  q.extend(['%s:%s' % (key, val) for key, val in params])
+  q.extend(['%s:%s' % (key, val.replace(" ", "+")) for key, val in params])
   return '+'.join(q)