subtree: use "$*" instead of "$@" as appropriate

subtree: use "$*" instead of "$@" as appropriate

"$*" is for when you want to concatenate the args together,
whitespace-separated; and "$@" is for when you want them to be separate
strings.

There are several places in subtree that erroneously use $@ when
concatenating args together into an error message.

For instance, if the args are argv[1]="dead" and argv[2]="beef", then
the line

    die "You must provide exactly one revision.  Got: '$@'"

surely intends to call 'die' with the argument

    argv[1]="You must provide exactly one revision.  Got: 'dead beef'"

however, because the line used $@ instead of $*, it will actually call
'die' with the arguments

    argv[1]="You must provide exactly one revision.  Got: 'dead"
    argv[2]="beef'"

This isn't a big deal, because 'die' concatenates its arguments together
anyway (using "$*").  But that doesn't change the fact that it was a
mistake to use $@ instead of $*, even though in the end $@ still ended
up doing the right thing.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>