Allow Specifying a Commit Message after a Squash

Description

When the squash merge happens the commit message is copied from the last commit on the branch being squashed. This leads to silly commit messages for the squash merge in mainline like "fixed something"

Proposal

it would be much better to either just describe the commit as "Squashed merge of branch blah" or simply let us edit the commit message before hitting the merge button.

Links / references

Zendesk Ticket: https://gitlab.zendesk.com/agent/tickets/67128

/cc @smcgivern