tests: protect tests involving git ext::sh with git-ext-sh

hghave: add has_git_range for testing if git understands ext::sh Even on CentOS 7, git is at version 1.8. It seems git 1.9 is when ext::sh was introduced so we a check for that. The way these functions are written follows the same style and format for the way we check svn and bzr versions.

rebase: rewrite "x in y.children()" as "y in x.parents()" children() is slow