repopick: Add a way to checkout instead of cherrypick

This helps if you want to take a commit and its dependencies, and not
just a particular commit

Change-Id: Ib245cce560c7b0d6fd03198a8c69a13d4720a4cb
add support for overriding build.prop settings.

Change-Id: Ie4224d608b7bcb0c2087bd77bc954f8f5f9b6495
build: work around missing readlink -f on Mac

Change-Id: I5d56366cf33a2b02f1886c87815d00cff279779d