py3: use encoding.environ in ui.py

Using source transformer we add b'' everywhere. So there are no chances
that those bytes string will work with os.environ on Py3 as that returns a
dict of unicodes. We are relying on the errors, even though no error is raised
even in future, these pieces of codes will tend to do wrong things.
if statements can result in wrong boolean and certain errors can be raised
while using this piece of code. Let's not wait for them to happen, fix what is
wrong. If this patch goes in, I will try to do it for all the cases.
Leaving it as it is buggy.

mercurial/ui.py

@@ -22,6 +22,7 b' from .node import hex'
 
 from . import (
     config,
+    encoding,
     error,
     formatter,
     progress,
@@ -569,9 +570,11 b' class ui(object):'
         - 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:
+        if ('HGPLAIN' not in encoding.environ and
+                'HGPLAINEXCEPT' not in encoding.environ):
             return False
-        exceptions = os.environ.get('HGPLAINEXCEPT', '').strip().split(',')
+        exceptions = encoding.environ.get('HGPLAINEXCEPT',
+                '').strip().split(',')
         if feature and exceptions:
             return feature not in exceptions
         return True
@@ -584,13 +587,13 b' class ui(object):'
         If not found and ui.askusername is True, ask the user, else use
         ($LOGNAME or $USER or $LNAME or $USERNAME) + "@full.hostname".
         """
-        user = os.environ.get("HGUSER")
+        user = encoding.environ.get("HGUSER")
         if user is None:
             user = self.config("ui", ["username", "user"])
             if user is not None:
                 user = os.path.expandvars(user)
         if user is None:
-            user = os.environ.get("EMAIL")
+            user = encoding.environ.get("EMAIL")
         if user is None and self.configbool("ui", "askusername"):
             user = self.prompt(_("enter a commit username:"), default=None)
         if user is None and not self.interactive():
@@ -814,9 +817,9 b' class ui(object):'
     def termwidth(self):
         '''how wide is the terminal in columns?
         '''
-        if 'COLUMNS' in os.environ:
+        if 'COLUMNS' in encoding.environ:
             try:
-                return int(os.environ['COLUMNS'])
+                return int(encoding.environ['COLUMNS'])
             except ValueError:
                 pass
         return util.termwidth()
@@ -1075,10 +1078,10 b' class ui(object):'
             editor = 'E'
         else:
             editor = 'vi'
-        return (os.environ.get("HGEDITOR") or
+        return (encoding.environ.get("HGEDITOR") or
                 self.config("ui", "editor") or
-                os.environ.get("VISUAL") or
-                os.environ.get("EDITOR", editor))
+                encoding.environ.get("VISUAL") or
+                encoding.environ.get("EDITOR", editor))
 
     @util.propertycache
     def _progbar(self):