git: use stash 'push' instead of the deprecated 'save' (#7486)

* Use stash 'push' or 'save' depending on Git version

A utility function now parses the output of git --version and set the
alias for git stash to 'git stash push' iff the current version of Git
is 2.13 or greater; it falls back to 'git stash save' otherwise.