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

Links / references

• https://git-scm.com/docs/git-rebase#git-rebase---autosquash
• https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History