Honor "squash!" in merge requests
Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/212
When using --autorebase squash! commits behave differently to fixup! commits because a squash combines two commits changes including commit message, and should allow the person squashing the commits to review and modify the commit message.
We should support both in a manner consistent with Git.
Proposal
If a commit contains squash! commits, the user should be shown a prompt to review the commit messages of the commits that will be squashed and be able edit them.
Example:
# This is a combination of 3 commits.
# The first commit's message is:
changed my name a bit
# This is the 2nd commit message:
updated README formatting and added blame
# This is the 3rd commit message:
added cat-file