Buildbot crashes on non-ascii characters in Git branch names

Had a developer send a PR that had the branch name `feature/🦆` which caused buildbot to crash when attempting to clone it.

Got this error:
```
buildbot.process.remotecommand.RemoteException: b'builtins.UnicodeEncodeError': 'ascii' codec can't encode character '\U0001f986' in position 4: ordinal not in range(128)
Traceback (most recent call last):
File "/usr/local/lib/python3.6/dist-packages/twisted/spread/pb.py", line 1054, in _recvMessage netResult = object.remoteMessageReceived(self, message, netArgs, netKw)
File "/usr/local/lib/python3.6/dist-packages/twisted/spread/flavors.py", line 125, in remoteMessageReceived state = method(*args, **kw)
File "/usr/local/lib/python3.6/dist-packages/buildbot_worker/base.py", line 153, in remote_startCommand d = self.command.doStart() File "/usr/local/lib/python3.6/dist-packages/buildbot_worker/commands/base.py", line 160, in doStart d = defer.maybeDeferred(self.start) --- ---
File "/usr/local/lib/python3.6/dist-packages/twisted/internet/defer.py", line 151, in maybeDeferred result = f(*args, **kw)
File "/usr/local/lib/python3.6/dist-packages/buildbot_worker/commands/shell.py", line 47, in start logEnviron=args.get('logEnviron', True),
File "/usr/local/lib/python3.6/dist-packages/buildbot_worker/runprocess.py", line 334, in __init__ self.command = to_bytes(util.Obfuscated.get_real(command))
File "/usr/local/lib/python3.6/dist-packages/buildbot_worker/runprocess.py", line 329, in to_bytes cmd[i] = a.encode(self.builder.unicode_encoding)

builtins.UnicodeEncodeError: 'ascii' codec can't encode character '\U0001f986' in position 4: ordinal not in range(128)
```

The traceback points to buildbot_worker/runprocess.py, especially to_bytes, after shell.py starts the command; begin there and reproduce the clone with a non-ASCII branch name. Done means the worker no longer crashes for that branch name and a regression test covers it.

Written by the indexing model from the issue text.