Allow specifying a custom commit message for @bors squash

[Kobzol]

By defaut, it will now include the messages of all squashed commits. The commit message can now be overwritten using @bors squash msg="Foo bar".

[programmerjake]

should this support escaping quotes?

[Kobzol]

Could be done as a follow-up, but I'd wait for a use-case 😆 It also can't deal with multi-line commit messages, backticks, etc., so it is limited anyway.

[programmerjake]

well, you're depending on serde_json anyway, why not use that to parse the string? something like:

// Check if value starts with quote
if i < len && bytes[i] == b'"' {
    let mut de = serde_json::de::Deserializer::from_slice(&bytes[i..]).into_iter();
    match String::deserialize(&mut de) {
        Ok(value) => {
            i += de.byte_offset();
            // handle string
        }
        Err(e) => {
            // handle error
        }
    }
}

[Kobzol]

Good idea! I tried it, but sadly it looks like pulldown_cmark removes the quote escapes :( The interaction between our parser and Markdown is a bit complicated (#430 (comment)).