tests/test-issue619.t

color: add a 'ui.color' option to control color behavior

This new option control whether or not color will be used. It mirror the behavior
of '--color'. I usually avoid adding new option to '[ui]' as the section is
already filled with many option. However, I feel like 'color' is central enough
to deserves a spot in this '[ui]' section.

For now the option is not documented so it is still marked as experimental. Once
it get documented and official, we should be able to deprecate the color
extensions.

There is more cleanup to do before that documentation is written, but we need
this option early to made them. Having that option will allow for more cleanup
of the initialisation process and proper separation between color
configuration.

https://bz.mercurial-scm.org/619

  $ 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]

Even with strange revset (issue4465)

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