Add `--is-draft` argument on patchset-created hook

The `--is-draft` argument will be passed with either `true` if
the patchset is a draft, or `false` otherwise.

This can be used by hooks that need to behave differently if the
change is a draft.

Change-Id: Idd00efe799bb05b0fd00e917faaccd5c772b1e1f
2 files changed