Suppose you like to write elaborate commit messages, and are committing to a branch that already has a corresponding phabricator review (created with arc diff --create). arc diff --update DXXXXX will start with a default message consisting of only the first line of each new commit, meaning any detailed explanations you wrote in a commit message are lost unless you rewrite them or go copy them from the commit log.

Ideally arc diff --update should by default preserve the full commit message