repo.bash: replace with better version

From https://raw.github.com/aartamonau/repo.bash_completion/master/repo.bash_completion

Change-Id: I7ca5d7eba70244d875c48ce76c999c8a10b88934
repo bash completion: _find_repo: improve check

Checking if main.py exists is better than checking if it
is a file, since it might be a symlink.

Change-Id: I1a9e617470a05c2161479cd3f0a10712a2b9d2c2