Encode the `name` parameter to bytes (using the utf8 codec).
This fixes the `TypeError` in subsequent concatenations in `get_branch`:
```
Traceback (most recent call last):
# stack omitted for brevity
File "C:\Dev\git-migration\fast-export\hg2git.py", line 73, in get_branch
return origin_name + b'/' + name
TypeError: can only concatenate str (not "bytes") to str
```
The conversion is done unconditionally since the passed
parameter is currently always of type `str`.
