i18n: guard against non-unicode results from gettext

Some configurations may end up with non-unicode results from the gettext
library.  Decode our inputs to ensure that they're always unicode.

Closes #969
Reported-by: Guo Yunhe <guoyunhebrave@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>

cola/i18n.py

@@ -19,6 +19,7 @@ def gettext(s):
         # Python 3 compat
         _translation.ugettext = _translation.gettext
         txt = _translation.gettext(s)
+    txt = core.decode(txt)
     if txt[-6:-4] == '@@':  # handle @@verb / @@noun
         txt = txt[:-6]
     return txt

share/doc/git-cola/relnotes.rst

@@ -18,6 +18,9 @@ Clone the git-cola repo to get the latest development version:
 
 Fixes
 -----
+* Non-unicode results from the `gettext` library are more robustly handled.
+  (`#969 <https://github.com/git-cola/git-cola/issues/969>`_)
+
 * Launching `git cola` from within a directory that has since been deleted
   would previously result in a traceback, and is now robustly handled.
   (`#961 <https://github.com/git-cola/git-cola/issues/961>`_)