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" <[e-mail]>

mercurial/ui.py

@@ -330,7 +330,7 b' class ui(object):'
             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 b' class ui(object):'
         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 feature and exceptions:
+            return feature not in exceptions
+        return True
 
     def username(self):
         """Return default username to be used in commits.

tests/test-hgrc.t

@@ -144,7 +144,7 b' plain mode with exceptions'
   $ echo "plain=./plain.py" >> $HGRCPATH
   $ HGPLAINEXCEPT=; export HGPLAINEXCEPT
   $ hg showconfig --config ui.traceback=True --debug
-  plain: ['']
+  plain: True
   read config from: $TESTTMP/hgrc
   $TESTTMP/hgrc:15: extensions.plain=./plain.py
   none: ui.traceback=True
@@ -153,7 +153,7 b' plain mode with exceptions'
   none: ui.quiet=False
   $ unset HGPLAIN
   $ hg showconfig --config ui.traceback=True --debug
-  plain: ['']
+  plain: True
   read config from: $TESTTMP/hgrc
   $TESTTMP/hgrc:15: extensions.plain=./plain.py
   none: ui.traceback=True
@@ -162,7 +162,7 b' plain mode with exceptions'
   none: ui.quiet=False
   $ HGPLAINEXCEPT=i18n; export HGPLAINEXCEPT
   $ hg showconfig --config ui.traceback=True --debug
-  plain: ['i18n']
+  plain: True
   read config from: $TESTTMP/hgrc
   $TESTTMP/hgrc:15: extensions.plain=./plain.py
   none: ui.traceback=True