Allow subcommand.color and color.subcommand color configuration

Allow subcommand.color and color.subcommand color configuration

While adding colour to the branch command it was pointed out that a
config option like "branch.color" conflicts with the pre-existing
"branch.something" namespace used for specifying default merge urls and
branches.  The suggested solution was to flip the order of the
components to "color.branch", which I did for colourising branch.

This patch does the same thing for
  - git-log (color.diff)
  - git-status (color.status)
  - git-diff (color.diff)
  - pager (color.pager)

I haven't removed the old config options; but they should probably be
deprecated and eventually removed to prevent future namespace
collisions.  I've done this deprecation by changing the documentation
for the config file to match the new names; and adding the "color.XXX"
options to contrib/completion/git-completion.bash.

Unfortunately git-svn reads "diff.color" and "pager.color"; which I
don't like to change unilaterally.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>