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 5bdece0 (gc/repack: release
packs when needed, 2018-12-15).

This fixes #3336.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

builtin/pull.c

@@ -1045,6 +1045,7 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 			oidclr(&rebase_fork_point);
 	}
 
+	close_object_store(the_repository->objects);
 	if (run_fetch(repo, refspecs))
 		return 1;