clean up name allocation in prepare_revision_walk

clean up name allocation in prepare_revision_walk

When we enter prepare_revision_walk, we have zero or more
entries in our "pending" array. We disconnect that array
from the rev_info, and then process each entry:

  1. If the entry is a commit and the --source option is in
     effect, we keep a pointer to the object name.

  2. Otherwise, we re-add the item to the pending list with
     a blank name.

We then throw away the old array by freeing the array
itself, but do not touch the "name" field of each entry. For
any items of type (2), we leak the memory associated with
the name. This commit fixes that by calling object_array_clear,
which handles the cleanup for us.

That breaks (1), though, because it depends on the memory
pointed to by the name to last forever. We can solve that by
making a copy of the name. This is slightly less efficient,
but it shouldn't matter in practice, as we do it only for
the tip commits of the traversal.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>