Show usage string for 'git unpack-file -h'

Show usage string for 'git unpack-file -h'

"unpack-file -h" could be asking to save the contents of a blob
named "-h".  Strictly speaking, such a pathological ref name is
possible, but the user would have to had said something like
"tags/-h" to name such a pathological ref already.  When used in
scripts, unpack-file is typically not passed a user-supplied tag
name directly.

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