specify that arg to `git-lg` is a branch, not file

fixes #30
Merge pull request #31 from ryan-williams/lg

specify that arg to `git-lg` is a branch, not file
add success message to end of run-tests script
trap EXIT, clean up test files in test scripts

fixes #32
set GIT_HELPERS_HOME in mk-git-test

allows this test to run correctly from an alternate copy of the
git-helpers repo, other than the one at $GIT_HELPERS_HOME.
add comment and extra checks to mk-git-test
fix overwrite-prompt logic in `mk-git`, add test