[PATCH 1 of 3] ui: test plain mode against exceptions

[Yann E. MORIN]

# HG changeset patch
# User "Yann E. MORIN" <yann.morin.1998 at anciens.enib.fr>
# Date 1305583731 -7200
# Node ID 2e0bfc555cc8395dfbc5e59c8e036266655dc77f
# Parent  abaacdab38f64e3f7f27d8bc6ec251c76dfcc30e
ui: test plain mode against exceptions

Let ui.plain() accept an optional parameter in the form of a feature
name (as a string) to exclude from plain mode.

The result of ui.plain is now:
 - False if HGPLAIN is not set or the requested feature is in HGPLAINEXCEPT
 - True otherwise

Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at anciens.enib.fr>

diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -330,7 +330,7 @@
             for name, value in self.configitems(section, untrusted):
                 yield section, name, value
 
-    def plain(self):
+    def plain(self, feature="NONE"):
         '''is plain mode active?
 
         Plain mode means that all configuration variables which affect
@@ -341,14 +341,16 @@
         The only way to trigger plain mode is by setting either the
         `HGPLAIN' or `HGPLAINEXCEPT' environment variables.
 
-        The return value can either be False, True, or a list of
-        features that plain mode should not apply to (e.g., i18n,
-        progress, etc).
+        The return value can either be
+        - False if HGPLAIN is not set, or feature is in HGPLAINEXCEPT
+        - True otherwise
         '''
         if 'HGPLAIN' not in os.environ and 'HGPLAINEXCEPT' not in os.environ:
             return False
         exceptions = os.environ.get('HGPLAINEXCEPT', '').strip().split(',')
-        return exceptions or True
+        if exceptions:
+            return feature not in exceptions
+        return True
 
     def username(self):
         """Return default username to be used in commits.