upload: add support for setting hashtags

This allows users to specify custom hashtags when uploading, both via
the CLI and via the same gitconfig settings as other upload options.

Bug: https://crbug.com/gerrit/11174
Change-Id: Ia0959e25b463e5f29d704e4d06e0de793d4fc77c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255855
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>