[ alias ]

    ignored = ! git ls-files --others --ignored --exclude-standard --directory \\
            && git ls-files --others -i --exclude-standard

Shows one line per file, so you can grep (only directories):

$ git ignored | grep '/$'
.yardoc/
doc/

Or count:

$ git ignored | wc -l
199811                 # oops, my home directory is getting crowded