builtin/checkout: compute checkout metadata for checkouts

builtin/checkout: compute checkout metadata for checkouts

Provide commit metadata for checkout code paths that use unpack_trees
and friends.  When we're checking out a commit, use the commit
information, but don't provide commit information if we're checking out
from the index, since there need not be any particular commit associated
with the index, and even if there is one, we can't know what it is.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>