unpack-trees: pass checkout state explicitly to check_updates()

unpack-trees: pass checkout state explicitly to check_updates()

Add a parameter for the struct checkout variable to check_updates()
instead of using a static global variable.  Passing it explicitly makes
object ownership and usage more easily apparent.  And we get rid of a
static variable; those can be problematic in library-like code.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>