Fix bug in finding remotes where _find_remote would return the remote name including the newline character which made self._rev_list_youngest_remote_ancestor fail.
This is due to str.splitlines not dropping the '\n' after splitting the lines.

Bug in _find_remote
rbt post -u --parent 9902a1599041080763bcf0d3fe610019a56d2c6f -r 1863684
> /Users/jolson/repos/svin/build/svin/venv/lib/python3.7/site-packages/rbtools/clients/git.py(1285)_find_remote()
-> if len(all_remotes) >= 1:
(Pdb) all_remotes
['origin\n']
(Pdb) self._execute(['git', 'remote'])
'origin\n'
(Pdb) self._execute(['git', 'remote'], split_lines=True)
['origin\n']