tests/test-mactext.t

documentation: add some internals documentation about bid merge

This is an important algorithm that was only documented on the wiki so far.

Some update to the algorithm (and associated doc) is to expected in the future
since the bid merge algorithm is bug-ridden when it comes to file deletion comes
to play.

Differential Revision: https://phab.mercurial-scm.org/D8711

  $ cat > unix2mac.py <<EOF
  > import sys
  > 
  > for path in sys.argv[1:]:
  >     data = open(path, 'rb').read()
  >     data = data.replace(b'\n', b'\r')
  >     open(path, 'wb').write(data)
  > EOF
  $ hg init
  $ echo '[hooks]' >> .hg/hgrc
  $ echo 'pretxncommit.cr = python:hgext.win32text.forbidcr' >> .hg/hgrc
  $ echo 'pretxnchangegroup.cr = python:hgext.win32text.forbidcr' >> .hg/hgrc
  $ cat .hg/hgrc
  [hooks]
  pretxncommit.cr = python:hgext.win32text.forbidcr
  pretxnchangegroup.cr = python:hgext.win32text.forbidcr

  $ echo hello > f
  $ hg add f
  $ hg ci -m 1

  $ "$PYTHON" unix2mac.py f
  $ hg ci -m 2
  attempt to commit or push text file(s) using CR line endings
  in dea860dc51ec: f
  transaction abort!
  rollback completed
  abort: pretxncommit.cr hook failed
  [255]
  $ hg cat f | f --hexdump
  
  0000: 68 65 6c 6c 6f 0a                               |hello.|
  $ f --hexdump f
  f:
  0000: 68 65 6c 6c 6f 0d                               |hello.|