tests/test-update-issue1456.t

serve: don't send any content headers with 304 responses

Fixes HTTP protocol violation introduced in cf5c76017e11. 'hg serve' would show
a stacktrace when loading pages that not had been modified.

There was test coverage for this, but the wrong response headers wasn't shown
and thus not detected.

  $ "$TESTDIR/hghave" execbit || exit 80

  $ rm -rf a
  $ hg init a
  $ cd a

  $ echo foo > foo
  $ hg ci -qAm0
  $ chmod +x foo
  $ hg ci -m1
  $ hg co -q 0
  $ echo dirty > foo
  $ hg up -c
  abort: uncommitted local changes
  [255]
  $ hg up -q
  $ cat foo
  dirty
  $ hg st -A
  M foo

Validate update of standalone execute bit change:

  $ hg up -C 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ chmod -x foo
  $ hg ci -m removeexec
  nothing changed
  [1]
  $ hg up -C 0
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg up
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg st

  $ cd ..