This seems like it should be a simple display preference...
I use Gitk to browse a repo and find an interesting commit. The full 40 char commit/SHA1 hash of the current commit appears under the list of commits:
How do I get the short form, i.e. what git log -n 1 --oneline
would generate? I often don't want to put the whole commit hash into for-human communications. It's ugly, and when you inevitably end up in situations where you can't copy/paste, it creates the unnecessary question of how many characters do you read off to guarantee uniqueness. It's also a bit of an odd discussion to have over the telephone. :-)
Edit > Preferences > General > Auto-select SHA1 (length) had no perceivable effect when changing the value from 40 to 10 or by turning off the checkbox:
Knowing the full hash is often of no value to me; all I need is uniqueness, and if the short hash is effectively unique, then I'm happy to not see the full hash in any circumstance that matters to how I use it.