color: turn on by default (but for windows)

Color support is all in core for a couple of months. I've browsed the bug tracker
without finding any blocker bug. So I'm moving forward and enable color on by
default before '4.2-rc'. In the worse case, having it on in the release
candidate will help us to find blocker bug and we can turn it off for the final
release.

I remember people talking about issue with Windows during the freeze so I'm
keeping it off by default on that OS.

We could do various cleaning of the color used and the label issued.  However
the label are probably already in our backward compatibility envelope since the
color extensions has been around since for ever and I do not think the color
choice themself should be considered BC. So I think we should rather gives color
to all user sooner than later.

A couple of test needs to be updated to avoid having color related control code
spoil the tested output.

mercurial/color.py

@@ -45,8 +45,7 b' except ImportError:'
     curses = None
     _baseterminfoparams = {}
 
-# allow the extensions to change the default
-_enabledbydefault = False
+_enabledbydefault = pycompat.osname != 'nt'
 
 # start and stop parameters for effects
 _effects = {

tests/test-mq.t

@@ -365,10 +365,10 b' pop/push -a in subdir'
 
 setting columns & formatted tests truncating (issue1912)
 
-  $ COLUMNS=4 hg qseries --config ui.formatted=true
+  $ COLUMNS=4 hg qseries --config ui.formatted=true --color=no
   test.patch
   test2.patch
-  $ COLUMNS=20 hg qseries --config ui.formatted=true -vs
+  $ COLUMNS=20 hg qseries --config ui.formatted=true -vs --color=no
   0 A test.patch: f...
   1 A test2.patch: 
   $ hg qpop

tests/test-pager-legacy.t

@@ -10,6 +10,7 b' pager was running.'
   $ cat >> $HGRCPATH <<EOF
   > [ui]
   > formatted = yes
+  > color = no
   > [extensions]
   > pager=
   > [pager]

tests/test-pager.t

@@ -10,6 +10,7 b' pager was running.'
   $ cat >> $HGRCPATH <<EOF
   > [ui]
   > formatted = yes
+  > color = no
   > [pager]
   > pager = python $TESTTMP/fakepager.py
   > EOF