Prevent stamping a commit if there are pending changes in the tree.
Prevent stamping a commit if there are pending changes in the tree.

When running rbt stamp with any pending changes in the index, those
changes could end being amended into the commit. We now check for
pending changes before attempting to stamp.

Reviewed at https://reviews.reviewboard.org/r/6712/

davidt/rbtools,datjwu/rbtools,beol/rbtools,davidt/rbtools,haosdent/rbtools,reviewboard/rbtools,haosdent/rbtools,reviewboard/rbtools,halvorlu/rbtools,beol/rbtools,datjwu/rbtools,halvorlu/rbtools,reviewboard/rbtools,haosdent/rbtools,davidt/rbtools,halvorlu/rbtools,beol/rbtools,datjwu/rbtools