pull: release packs before fetching
On Windows, files cannot be removed nor renamed if there are still handles held by a process. To remedy that, we try to release all open handles to any `.pack` file before e.g. repacking (which would want to remove the original `.pack` file(s) after it is done). Since the `read_cache_unmerged()` and/or the `get_oid()` call in `git pull` can cause `.pack` files to be opened, we need to release the open handles before calling `git fetch`: the latter process might want to spawn an auto-gc, which in turn might want to repack the objects. This commit is similar in spirit to 5bdece0d705 (gc/repack: release packs when needed, 2018-12-15). This fixes https://github.com/git-for-windows/git/issues/3336. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>