Merge branch 'ow/rev-parse-is-shallow-repo'

Merge branch 'ow/rev-parse-is-shallow-repo'

"git rev-parse" learned "--is-shallow-repository", that is to be
used in a way similar to existing "--is-bare-repository" and
friends.

* ow/rev-parse-is-shallow-repo:
  rev-parse: rev-parse: add --is-shallow-repository