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>

packfile.c

@@ -16,6 +16,7 @@
 #include "tree.h"
 #include "object-store.h"
 #include "midx.h"
+#include "commit-graph.h"
 
 char *odb_pack_name(struct strbuf *buf,
 		    const unsigned char *sha1,
@@ -350,6 +351,8 @@ void close_all_packs(struct raw_object_store *o)
 		close_midx(o->multi_pack_index);
 		o->multi_pack_index = NULL;
 	}
+
+	close_commit_graph(o);
 }
 
 /*