Most issues are on grit side but basically to make it work on Windows in a msysgit I had to:

• Install git-up (like described)
• Patch lib/grit/git.rb (see wernight/grit@bdc23bd)
• Patch lib/posix/spawn.rb (see stereobooster/posix-spawn@67a73c6)
• Use ansicon to get colors: $ ansicon git up

Possibly using https://github.com/schacon/grit would works without most of those patches.Using another coloring system in place of the current one would be advisable as it is deprecated.