Add more checkout tests

Add more checkout tests

If you have local changes that don't conflict with the
branch-switching changes, these should be kept, not cause errors even
without -m, and be reported afterwards in name-status format.

With -m, the changes carried across should be listed as well. And, for
now, include the merge-recursive output from this process.

Also test the detatched head message in at least one case.

Signed-off-by: Daniel Barkalow <redacted>
Signed-off-by: Junio C Hamano <redacted>