Render MR suggestion's placeholder commit message without variables

The following discussion from !48597 (merged) should be addressed:

• @pedroms started a discussion: (+5 comments)

In !48597 (merged) we added the ability to define a custom commit message when applying a suggestion.

Currently, the placeholder for the textarea can contain variables such as %{suggestions_count}, %{file_paths}, %{branch_name} etc. that come from the default commit message for suggestions.

Ideally, we should be using the resolved default commit message as a placeholder:

Current: Apply suggestion for %{project_name} on %{file_paths}

The current placeholder with the variables can look intimidating, or worse, it can look like a bug. It's also not representative of the commit message that will be used when applying the suggestion.

Expected: Apply suggestion for my-project on my-file.rb