tests/test-issue619.t

diff: move b85diff to mdiff module

b85diff generates a binary diff, so we move this code to mdiff module
along with unidiff for text diffs. All diffing mechanisms will be in the
same place.

In an upcoming patch we will remove the responsibility to print the
index header from b85diff and move it back to patch, since it's
a patch metadata header, not part of the diff generation.

http://mercurial.selenic.com/bts/issue619

  $ hg init
  $ echo a > a
  $ hg ci -Ama
  adding a

  $ echo b > b
  $ hg branch b
  marked working directory as branch b
  (branches are permanent and global, did you want a bookmark?)
  $ hg ci -Amb
  adding b

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

Fast-forward:

  $ hg merge b
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -Ammerge

Bogus fast-forward should fail:

  $ hg merge b
  abort: merging with a working directory ancestor has no effect
  [255]