The local tag feature was intended to make phabsend closer to email
workflow. But its experience is not great in multiple ways:
- after rebase, obsoleted changesets are still visible because of tags
- without obsstore, the association information will get lost
- even with obsstore, things could go wrong with graft, export+import
- no easy way to tell which Differential Revision a commit is associated
Therefore make --amend the default. People wanting the old behavior can
use --no-amend.