setup: split off get_device_or_die helper
setup: split off get_device_or_die helper

This does not eliminate any code, but it skims some off of
the main loop of setup_git_directory_gently so that can be
understood more easily.

Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git