bundle: keep around names passed to add_pending_object()

The 'name' field passed to add_pending_object() is used to later
deduplicate in object_array_remove_duplicates().

git-bundle had a bug in this area since 18449ab0 (git-bundle: avoid
packing objects which are in the prerequisites, 2007-03-08): it passed
the name of each boundary object in a static buffer.  In other words,
all that object_array_remove_duplicates() saw was the name of the
*last* added boundary object.

The recent switch to a strbuf in bc2fed49 (bundle: use a strbuf to scan
the log for boundary commits, 2012-02-22) made this slightly worse: we
now free the buffer at the end, so it is not even guaranteed that it
still points into addressable memory by the time object_array_remove_
duplicates looks at it.  On the plus side however, it was now
detectable by valgrind.

The fix is easy: pass a copy of the string to add_pending_object.

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