Fix add selected hunk with non-ascii characters 

Add selected hunk fails if the hunk contains non ascii characters.
Adding `encode('utf-8')` fixes the error.

```
Exception in thread Thread-353:
Traceback (most recent call last):
  File ".\threading.py", line 532, in __bootstrap_inner
  File ".\git.py", line 122, in run
  File ".\subprocess.py", line 701, in communicate
  File ".\subprocess.py", line 911, in _communicate
UnicodeEncodeError: 'ascii' codec can't encode characters in position 372-373: ordinal not in range(128)
```

I also noticed the issue #181 and fixed it.