diff headers should contain non-ascii filenames in user_encoding, not in utf-8

[Alexander Belchenko]

Currently bzr can produce diff as result of 5 different operations:

bzr diff
bzr commit --show-diff
bzr log -p
bzr merge --preview
bzr send

In most of these commands non-ascii filename always shown as utf-8 string. This is bad for windows users, because their locale never utf-8 (at least by default), and not always it's possible to switch console locale to utf-8 (chcp 65001 won't work on all Windows versions).
Furthermore, bzr itself does not understand cp65001 codepag because Python does not recognize it as utf-8, see http://bugs.python.org/issue6058.

I think all comands except `send` should always print filenames in user_encoding, or AT LEAST show them in user_encoding in the === line (as bzr ci --show-diff currently does). Because all these commands are intended to produce output for humans.

Also, I should note that GNU diff (from http://gnuwin32.sf.net) always print filenames in user_encoding.

See attached files with output of various commands.

Will be nice to fix this before 2.0. Some guidance needed from core devs, especially about writing tests for this changes.