Vanilla git has a git-send-email(1) subcommand. `git cl upload` should
follow suit by using the same flag name to be consistent and avoid
confusion.

[1]: https://git-scm.com/docs/git-send-email

Bug: none
Change-Id: I47ff8e03bd11915e99804fe1695046d874efc4e0
Tested: `git cl upload --send-email` to upload this very own CL
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3854330Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Auto-Submit: Thiago Perrotta <tperrotta@chromium.org>