Many users in my organization want to branch off of a branch. As described in https://secure.phabricator.com/book/phabricator/article/arcanist_diff/ and https://secure.phabricator.com/book/phabricator/article/arcanist_commit_ranges/, the default base for commit ranges doesn't support this well, and git:branch-unique has some weird edge cases.
This problem is difficult to solve automatically for all users and use cases, but it wouldn't be an issue at all if users knew that they could use arc diff --base other-branch. However, since most of the time a normal arc diff does the right thing, it's hard to form a habit.
As such, it would be great to add information in the comment in the differential update comment educating users about --base. Adding something like the following in arcanist/src/workflow/ArcanistDiffWorkflow.php:getUpdateMessage would do the trick:
If this is a sub-feature (eg, a branch of a branch) and you don't want to update this revision, you can use: arc diff --base parent-branch
If it wouldn't be desirable for all users to have that as part of the update message, it would be great if users could have custom update templates so that the default could stay the same but organizations that like branches off branches like mine could add that help text.