Update the virtualfilesystem support

We now specify that it needs to be run from the root of the git work
tree. This enables the hook to be found even if the current working
directory is not the root of the repo (like when running 'git diff' with
Beyond Compare configured as the diff tool.

Also simpify how argv[] parameter is created.

Signed-off-by: Ben Peart <benpeart@microsoft.com>

virtualfilesystem.c

@@ -32,18 +32,14 @@ static int vfs_hashmap_cmp(const void *unused_cmp_data,
 static void get_virtual_filesystem_data(struct strbuf *vfs_data)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
-	char ver[64];
-	const char *argv[3];
 	int err;
 
 	strbuf_init(vfs_data, 0);
 
-	snprintf(ver, sizeof(ver), "%d", HOOK_INTERFACE_VERSION);
-	argv[0] = core_virtualfilesystem;
-	argv[1] = ver;
-	argv[2] = NULL;
-	cp.argv = argv;
+	argv_array_push(&cp.args, core_virtualfilesystem);
+	argv_array_pushf(&cp.args, "%d", HOOK_INTERFACE_VERSION);
 	cp.use_shell = 1;
+	cp.dir = get_git_work_tree();
 
 	err = capture_command(&cp, vfs_data, 1024);
 	if (err)