pack-objects: remove SP at the end of usage string

These spaces immediately before the end of lines are unnecessary.

While at it, instead of using a single string literal with backslashes
at end of each line, split the lines into individual string literals
and tell the compiler to concatenate them.

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>