Pass the users remote branch to git request-pull

Git 1.x tired to guess the right remote branch by trying to match the
local HEAD sha with a commit on the remote which could result into a
wrong guess.

Git 2.x fixed this by emitting `warn:` messages and exiting with code 2.

see http://git.661346.n2.nabble.com/2-0-0-regression-request-pull-does-not-seem-to-find-head-td7612178.html