Previously hg phabsend was imitating hg email and won't mutate
changesets. That works fine with reviewer-push workflow, reviewers run
However, it does not work well with author-push workflow. Namely, the author
needs to run extra commands to get the right commit message, and remove the
local tag after push.
This patch solves those issues by adding the --amend option, so local
changesets will have the right commit message, and tags become unnecessary.