$ hg init

  $ echo This is file a1 > a

  $ echo This is file b1 > b

  $ hg add a b

  $ hg commit -m "commit #0"

  $ echo This is file b22 > b

  $ hg commit -m "comment #1"

  $ hg update 0

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

  $ rm b

  $ hg commit -A -m "comment #2"

  removing b

  created new head

  $ hg update 1

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

  $ hg update

  abort: not a linear update

  (merge or update --check to force update)

  [255]

  $ rm b

  $ hg update -c

  [255]

  $ hg revert b

  $ hg update -c

  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ mv a c

In theory, we shouldn't need the "-y" below, but it prevents this test

from hanging when "hg update" erroneously prompts the user for "keep

or delete".

Should abort:

  $ hg update -y 1

  abort: uncommitted changes

  (commit or update --clean to discard changes)

  [255]

  $ mv c a

Should succeed:

  $ hg update -y 1

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved