This is similar in spirit to the helper flags in git subcommands, e.g.
try `git branch --git-completion-helper`. It provides a list of long
flags that the subcommands support, without repeating it manually in the
completion script. Note that this special flag is not returned in help,
so there is no noise.

To see the result, source the script, then `git cl upload --<TAB>`.

Change-Id: I82e280ceac5178b0e5fa52611b9a4125e6943337
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2209926Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>