bisect: add support for bisecting bare repositories

bisect: add support for bisecting bare repositories

This enhances the support for bisecting history in bare repositories.

The "git bisect" command no longer needs to be run inside a repository
with a working tree; it defaults to --no-checkout when run in a bare
repository.

Two tests are included to demonstrate this behaviour.

Suggested-by: Junio C Hamano <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Jon Seymour <redacted>
Signed-off-by: Junio C Hamano <redacted>