phabsend: make --amend the default

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`.

Differential Revision: https://phab.mercurial-scm.org/D511

contrib/phabricator.py

@@ -7,9 +7,9 b''
 """simple Phabricator integration
 
 This extension provides a ``phabsend`` command which sends a stack of
-changesets to Phabricator without amending commit messages, and a ``phabread``
-command which prints a stack of revisions in a format suitable
-for :hg:`import`, and a ``phabupdate`` command to update statuses in batch.
+changesets to Phabricator, and a ``phabread`` command which prints a stack of
+revisions in a format suitable for :hg:`import`, and a ``phabupdate`` command
+to update statuses in batch.
 
 By default, Phabricator requires ``Test Plan`` which might prevent some
 changeset from being sent. The requirement could be disabled by changing
@@ -339,7 +339,7 b' def userphids(repo, names):'
 
 @command('phabsend',
          [('r', 'rev', [], _('revisions to send'), _('REV')),
-          ('', 'amend', False, _('update commit messages')),
+          ('', 'amend', True, _('update commit messages')),
           ('', 'reviewer', [], _('specify reviewers')),
           ('', 'confirm', None, _('ask for confirmation before sending'))],
          _('REV [OPTIONS]'))