Add a tool that will tell which git projects are used for a build target

See build/make/tools/whichgit -h for the options

Test: build/make/tools/whichgit --modules com.google.android.tzdata
Test: build/make/tools/whichgit --modules framework
Test: TARGET_BUILD_APPS=com.google.android.tzdata4 build/make/tools/whichgit --products mainline_modules_arm64 --variants eng --modules $TARGET_BUILD_APPS --why "*"
Change-Id: I05d91b85575a920b3ec9f5959e2ce463a45e63a3