Currently multiple reviewers can only be provided with a comma-separated
list, for example: -r foo,bar,baz

This CL adds support for multiple -r arguments such as:

-r foo -r bar -r baz

Or even:

-r foo,bar -r baz

This makes it consistent (and thus less confusing) with `git cl upload` which does accept multiple -r arguments.

Bug: none
Change-Id: I27d03601b488c0c5b27568d86dcb4ed66df6b76f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3863675
Commit-Queue: Thiago Perrotta <tperrotta@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Auto-Submit: Thiago Perrotta <tperrotta@chromium.org>