Merge pull request #24 Match multi-pack-index feature from upstream

This includes commits that fixup!-revert all the midx-related commits from our GVFS branch and replaces them with the exact commits that are being merged upstream. This should automatically remove the commits during our next version rebase-and-merge action.

Changes upstream:
- The builtin is called 'git multi-pack-index'.
- The command-line takes a 'write' verb and an '--object-dir' parameter.
- We no longer have a 'midx-head' or '*.midx' files.
- Instead, we have a 'multi-pack-index' file in the pack-dir.
- It no longer makes sense to specify '--update-head'

builtin/fsck.c

@@ -49,6 +49,7 @@ static int name_objects;
 #define ERROR_PACK 04
 #define ERROR_REFS 010
 #define ERROR_COMMIT_GRAPH 020
+#define ERROR_MULTI_PACK_INDEX 040
 
 static const char *describe_object(struct object *obj)
 {
@@ -968,7 +969,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 			midx_argv[2] = "--object-dir";
 			midx_argv[3] = odb->path;
 			if (run_command(&midx_verify))
-				errors_found |= ERROR_COMMIT_GRAPH;
+				errors_found |= ERROR_MULTI_PACK_INDEX;
 		}
 	}