pack-write.c: plug a leak in stage_tmp_packfiles()

pack-write.c: plug a leak in stage_tmp_packfiles()

The function `stage_tmp_packfiles()` generates a filename to use for
staging the contents of what will become the pack's ".rev" file.

The name is generated in `write_rev_file_order()` (via its caller
`write_rev_file()`) in a string buffer, and the result is returned back
to `stage_tmp_packfiles()` which uses it to rename the temporary file
into place via `rename_tmp_packfiles()`.

That name is not visible outside of `stage_tmp_packfiles()`, so it can
(and should) be `free()`'d at the end of that function. We can't free it
in `rename_tmp_packfile()` since not all of its `source` arguments are
unreachable after calling it.

Instead, simply free() `rev_tmp_name` at the end of
`stage_tmp_packfiles()`.

(Note that the same leak exists for `mtimes_tmp_name`, but we do not
address it in this commit).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>