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/+/2832653Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>