Allow modifying squash commit message for fast-forward only merge method

What does this MR do?

Currently there is no possibility to edit a squash commit message when Fast-forward merge and Squash commits are both enabled. This MR allows user to edit squash commit message on fast-forward merge.

Possible cases

1. Fast-forward merge is enabled, there is only one commit in current MR. We don't show an expandable header and Squash commits checkbox, commit message is used for merging MR to target branch

2. Fast-forward merge is enabled, Squash commits checkbox is unchecked. We don't show an expandable header as all commits will to to target branch with respective commit messages

3. Fast-forward merge is enabled, Squash commits checkbox is checked. We show the collapsible header with **1 commit** will be added to master message. On expanding the header we show a field only for editing a squash commit message (because there will be no merge commit message):

Collapsed	Expanded

What are the relevant issue numbers?

#47149 (closed)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58631

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #48324 (closed)