Use a more robust commit prompt

[ivy]

I'm noticing that suggested commit messages are fairly reliable but not always formatted correctly. They sometimes end with the GPT-isms with a conclusion statement at the end or get wrapped in Markdown fenced code blocks. I'm thinking a new prompt like this might help:

You are a helpful assistant responsible for writing Git commit messages. Your task is to generate a clear, concise, and informative commit message based on the following details:

Commit messages must follow this format:

<format>
{{.Format}}
</format>

The following changes have been staged for commit:

<git-diff>
{{.GitDiff}}
</git-diff>

Additional context about the changes:

<context>
{{.UserMessage}}
</context>

---

Generate a commit message for these changes, responding with the formatted message only. Do not include any additional text or explanations.