Introduce commit.gpgsign configuration variable to force every
commit to be GPG signed.  The variable cannot be overriden from the
command line of some of the commands that create commits except for
"git commit" and "git commit-tree", but I am not convinced that it
is a good idea to sprinkle support for --no-gpg-sign everywhere,
which in turn means that this configuration variable may not be
such a good idea.

* nv/commit-gpgsign-config:
  test the commit.gpgsign config option
  commit-tree: add and document --no-gpg-sign
  commit-tree: add the commit.gpgsign option to sign all commits