tests/test-revlog-packentry.t

formatter: add helper to create a formatter optionally backed by file

To make things simple, openformatter() and maybereopen() have no support
for a plain object API. Callers must use the "with" statement.

Unlike cmdutil.makefileobj(), append mode ('ab') isn't supported by these
functions. This is because JSON output can't be simply concatenated. Perhaps
cmdutil.export() will have to build a {filename: [revs...]} map first and
write revs per file.

  $ hg init repo
  $ cd repo

  $ touch foo
  $ hg ci -Am 'add foo'
  adding foo

  $ hg up -C null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

this should be stored as a delta against rev 0

  $ echo foo bar baz > foo
  $ hg ci -Am 'add foo again'
  adding foo
  created new head

  $ hg debugindex foo
     rev    offset  length  ..... linkrev nodeid       p1           p2 (re)
       0         0       0  .....       0 b80de5d13875 000000000000 000000000000 (re)
       1         0      13  .....       1 0376abec49b8 000000000000 000000000000 (re)

  $ cd ..