cmds: force utf-8 when displaying untracked files

We do a partial read of untracked files which means that we may
sometimes read invalid utf-8 when the read is truncated.

Handle this in a more robust way by forcing utf-8 and skipping over
truncated bytes at the end of the read.

Closes #401
Reported-by: Ｖ字龍(Vdragon) <Vdragon.Taiwan@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>

cola/cmds.py

@@ -1334,7 +1334,8 @@ def diff_text_for(self, filename):
         cfg = gitcfg.current()
         size = cfg.get('cola.readsize', 1024 * 2)
         try:
-            result = core.read(filename, size=size)
+            result = core.read(filename, size=size,
+                               encoding='utf-8', errors='ignore')
         except:
             result = ''