entering a merge proposal commit message and description is tedious, redundant

[Martin Pool]

Merge proposals have fields for a description and proposed commit message, both of which are required for the typical mp workflow.

They do have distinct reasons to exist: the description is a letter to the people who will review the merge, and the commit message is (typically shorter) and a description of how it should be recorded in the branch history.

However, they typically also have a lot in common with each other and with the revisions committed to the branch. In the low-end case of "fix typo in foo_routine" people very likely want to use the same message for the original commit, the mp description, and the proposed commit message, but they have to manually copy it around.

This could be addressed by eg:
 * pre-filling the editor with something based on the to-be-merged revision messages
 * making the mp page and mail prominently show the to-be-merged commit messages, so the user only needs a description if they are not sufficient or are too long
 * making it clear the 'description' is only meant to be additional information beyond the proposed commit message
 * removing the 'description' and just having a commit message plus a conversation thread

fixing this might fix bug 619810