read_revisions_from_stdin: make copies for handle_revision_arg

read_revisions_from_stdin: make copies for handle_revision_arg

read_revisions_from_stdin() has passed pointers to its read buffer
down to handle_revision_arg() since its inception way back in 42cabc3
(Teach rev-list an option to read revs from the standard input.,
2006-09-05).  Even back then, this was a bug: through
add_pending_object, the argument was recorded in the object_array's
'name' field.

Fix it by making a copy whenever read_revisions_from_stdin() passes an
argument down the callchain.  The other caller runs handle_revision_arg()
on argv[], where it would be redundant to make a copy.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>