copybot: hashtag push option should be `t`, not `ht`

Copybot was passing the wrong push option key for hashtags. It should be
`t` or `hashtag` according to the documentation:
https://gerrit-review.googlesource.com/Documentation/user-upload.html#push_options

BUG=None
TEST=unit tests

Change-Id: I99d83dbc63ab9768d00683187590d7cf3c154202
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4849031
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Tristan Honscheid <honscheid@google.com>
Auto-Submit: Tristan Honscheid <honscheid@google.com>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Jonathon Murphy <jpmurphy@google.com>