Add --unused parameter to whichgit

The --unused parameter inverts the output of whichgit, reporting which
git projects are not used for a given build target.

Test: build/make/tools/whichgit --unused
Test: build/make/tools/whichgit --unused --modules framework

Existing use-cases should remain unchanged:

Test: build/make/tools/whichgit --modules framework
Change-Id: Ia4e55a5cb0331d522fed76821fe813ef98c25a67