color: respect HGPLAINEXCEPT=color to allow colors while scripting (issue5749)

I'd also like --color=always on the command-line to override HGPLAIN=1
et al, but that's more work, and this seems like a better fix. We've
got a fair number of programs that actually want to automate hg and
get colored output to users, so they should set HGPLAINEXCEPT=alias
(what we usually recommend), but this has been breaking them because
they then lose color.

.. feature::

  The ``HGPLAINEXCEPT`` environment variable can now include ``color``
  to allow automatic output colorization in otherwise automated
  environments.

Differential Revision: https://phab.mercurial-scm.org/D1532

mercurial/color.py

@@ -181,7 +181,7 b' def setup(ui):'
         configstyles(ui)
 
 def _modesetup(ui):
-    if ui.plain():
+    if ui.plain('color'):
         return None
     config = ui.config('ui', 'color')
     if config == 'debug':

mercurial/help/environment.txt

@@ -65,6 +65,8 b' HGPLAINEXCEPT'
 
     ``alias``
         Don't remove aliases.
+    ``color``
+        Don't disable colored output.
     ``i18n``
         Preserve internationalization.
     ``revsetalias``

tests/test-status-color.t

@@ -29,6 +29,22 b' hg status in repo root:'
   [status.unknown|? ][status.unknown|b/2/in_b_2]
   [status.unknown|? ][status.unknown|b/in_b]
   [status.unknown|? ][status.unknown|in_root]
+HGPLAIN disables color
+  $ HGPLAIN=1 hg status --color=debug
+  ? a/1/in_a_1
+  ? a/in_a
+  ? b/1/in_b_1
+  ? b/2/in_b_2
+  ? b/in_b
+  ? in_root
+HGPLAINEXCEPT=color does not disable color
+  $ HGPLAINEXCEPT=color hg status --color=debug
+  [status.unknown|? ][status.unknown|a/1/in_a_1]
+  [status.unknown|? ][status.unknown|a/in_a]
+  [status.unknown|? ][status.unknown|b/1/in_b_1]
+  [status.unknown|? ][status.unknown|b/2/in_b_2]
+  [status.unknown|? ][status.unknown|b/in_b]
+  [status.unknown|? ][status.unknown|in_root]
 
 hg status with template
   $ hg status -T "{label('red', path)}\n" --color=debug