export GIT_HELPERS_HOME, use it for `source`ing

fixes #27
source helpers in `clean_branches`, for `count` fn
add and use `prompt` helper

fixes #25, provides missing `clean_branches` dependency
more verbose error message in tests’ `check_link`
fix clean_branches shebang line
Merge pull request #28 from ryan-williams/home

fix `source`ing, add `prompt` helper
specify that arg to `git-lg` is a branch, not file

fixes #30