In git 2.9.0, commit-tree does not respect commit.gpgsign on its own,
which it used to.

So, in Git.Branch.commitTree, it needs to check if gpgsign is set,
and pass -S to commit-tree.

(commit-tree -S works in older versions of git, so this can
be done without version checks.)

--[[Joey]] 

> [[done]] --[[Joey]]
