tests/test-absorb-phase.t

help: explain in `hg help flags` that unambiguous prefixes are allowed

I used `hg commit --amend` as an example because that's the most
frequently used flag I could think of that doesn't yet have a short
form.

Differential Revision: https://phab.mercurial-scm.org/D8607

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > absorb=
  > drawdag=$TESTDIR/drawdag.py
  > EOF

  $ hg init
  $ hg debugdrawdag <<'EOS'
  > C
  > |
  > B
  > |
  > A
  > EOS

  $ hg phase -r A --public -q
  $ hg phase -r C --secret --force -q

  $ hg update C -q
  $ printf B1 > B

  $ hg absorb -aq

  $ hg log -G -T '{desc} {phase}'
  @  C secret
  |
  o  B draft
  |
  o  A public