"git commit-tree" plumbing command required the user to always sign
its result when the user sets the commit.gpgsign configuration
variable, which was an ancient mistake.  Rework "git rebase" that
relied on this mistake so that it reads commit.gpgsign and pass (or
not pass) the -S option to "git commit-tree" to keep the end-user
expectation the same, while teaching "git commit-tree" to ignore
the configuration variable.  This will stop requiring the users to
sign commit objects used internally as an implementation detail of
"git stash".

* jc/commit-tree-ignore-commit-gpgsign:
  commit-tree: do not pay attention to commit.gpgsign