feat(common.git): add `check_git_version` function
feat(common.git): add `check_git_version` function

This adds a util function (which is called on import) which checks for
missing or old versions of git and raises an appropriate warning.

This change includes tests.

larsbutler/simpl,checkmate/simpl,ziadsawalha/simpl,ryandub/simpl,samstav/simpl