packfile: close commit-graph in close_all_packs

The close_all_packs() method is used to close all read handles to pack-files and the multi-pack-index before running 'git gc --auto'. This is particularly important on the Windows platform, where read handles block any writes to those files. Replacing one of these files with a rename() will fail in this situation. The commit-graph also performs a rename, so is susceptable to this problem. We are careful to close the commit-graph before writing, but that doesn't work when a 'git fetch' (or similar) process runs 'git gc --auto' which may write a commit-graph. Here, close the commit-graph as part of close_all_packs(). Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>