repo: exit on missing entry point

exit if no repo_main can be found right before executing the command.
This happens for instance when 'repo init' is run on root path
(for example in a container). Without this counter measure the tool
will crash at exec_command with
TypeError: sequence item 1: expected str instance, NoneType found

Change-Id: Ia8480cfe2151c3b35c9572789ad8cb619288cce1
Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/263457


Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@digital.ai>