Add tip to reflog output showing example diff.namespace usage

[jesselooney]

Closes #3145

Running reflog in ucm now displays a tip below the list of history entries:

...
4. #sg60bvjo91 : ...

Tip: Use `diff.namespace 1 7` to compare namespaces between two points in history.

I copied the example in the issue verbatim, and I hope the wording is fine.

This is my first pull request, so let me know if there is anything I've done wrong.

[pchiusano]

Nice! Could you just add yourself to CONTRIBUTORS.markdown?

[pchiusano]

Also looking at CI, do a stack exec transcripts to refresh the transcripts. We have tests that exercise ucm interactions, and CI checks that the results haven't changed accidentally. Some of the transcripts use reflog so their output will change (as expected), but you need to run the transcripts and then check in all the changed transcripts as part of this PR.

Thanks again!

[jesselooney]

Done and done! Thanks for the info about the transcripts.

Also, I see a variety of branch names in the repository (e.g. 21-10-07-misc-cleanup, add-integration-tests, fix/1250, mitchell/use-pinboard). For any future PRs I might make, is there a preferred branch naming convention?

[jesselooney]

No problem, thanks!