subdirectories deprecated warning always shows and stop fetch

CB-11979 introduced a warning for using subdir with git url.
But even there is no subdir, it still show the warning, or prevent fetch.

Because the line of code `options.subdir = options.subdir || '.';`
And `if (result[2]) options.subdir = result[2];` will not set subdir to empty
Then `if(options.subdir) {` will always true since options.subdir is '.' by default.