[This is totally Mac-specific]
Rather late in life I'm learning to use source control, specifically #git . I did not want to learn vim on top of it, rather use the excellent #textwrangler edit my commit messages. The ever-resourceful Internet explains to you how to do this, e.g. here
but it didn't work for me. After rummaging around, I understood that the advice »… "install command line tools" from the drop down« does apply to versions of TextWrangler obtained via the Apple store. The solution was to visit http://www.barebones.com/support/textwrangler/faqs.html#cmdlinetools to download the tools. I installed them, and then in the terminal entered
git config --global core.editor "edit -w"
Now TextWrangler automatically opens to write the git commit messages. Nice.