* common.mk (mini): portable target to build miniruby

* common.mk (bisect): run git-bisect with miniruby

* common.mk (bisect-ruby): run git-bisect with ruby

* tool/bisect.sh: script for git-bisect

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42436 b2dd03c8-39d4-4d8f-98ff-823fe69b080e