i18n: use unicode literal

i18n: use unicode literal

Other parts of this expression are already using unicode literals.
We need this to make Python 3 happy and to avoid an implicit
conversion in Python 2.

--- a/mercurial/i18n.py	Sat Jun 25 17:22:06 2016 -0700
+++ b/mercurial/i18n.py	Sat Jun 25 17:40:53 2016 -0700
@@ -78,7 +78,7 @@
             paragraphs = [p.decode("ascii") for p in message.split('\n\n')]
         # Be careful not to translate the empty string -- it holds the
         # meta data of the .po file.
-        u = u'\n\n'.join([p and _ugettext(p) or '' for p in paragraphs])
+        u = u'\n\n'.join([p and _ugettext(p) or u'' for p in paragraphs])
         try:
             # encoding.tolocal cannot be used since it will first try to
             # decode the Unicode string. Calling u.decode(enc) really