$ hg init repo

  $ cd repo

  $ cat > .hg/hgrc <<EOF

  > [extensions]

  > prefixfilter = prefix.py

  > [encode]

  > *.txt = stripprefix: Copyright 2046, The Masters

  > [decode]

  > *.txt = insertprefix: Copyright 2046, The Masters

  > EOF

  $ cat > prefix.py <<EOF

  > from mercurial import error

  > def stripprefix(s, cmd, filename, **kwargs):

  >     header = b'%s\n' % cmd

  >     if s[:len(header)] != header:

  >         raise error.Abort(b'missing header "%s" in %s' % (cmd, filename))

  >     return s[len(header):]

  > def insertprefix(s, cmd):

  >     return b'%s\n%s' % (cmd, s)

  > def reposetup(ui, repo):

  >     repo.adddatafilter(b'stripprefix:', stripprefix)

  >     repo.adddatafilter(b'insertprefix:', insertprefix)

  > EOF

  $ cat > .hgignore <<EOF

  > .hgignore

  > prefix.py

  > prefix.pyc

  > __pycache__/

  > EOF

  $ cat > stuff.txt <<EOF

  > Copyright 2046, The Masters

  > Some stuff to ponder very carefully.

  > EOF

  $ hg add stuff.txt

  $ hg ci -m stuff

Repository data:

  $ hg cat stuff.txt

  Some stuff to ponder very carefully.

Fresh checkout:

  $ rm stuff.txt

  $ hg up -C

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

  $ cat stuff.txt

  Copyright 2046, The Masters

  Some stuff to ponder very carefully.

  $ echo "Very very carefully." >> stuff.txt

  $ hg stat

  M stuff.txt

  $ echo "Unauthorized material subject to destruction." > morestuff.txt

Problem encoding:

  $ hg add morestuff.txt

  $ hg ci -m morestuff

  abort: missing header "Copyright 2046, The Masters" in morestuff.txt

  [255]

  $ hg stat

  M stuff.txt

  A morestuff.txt