GHSA-4j89-2c4f-44c6

### Summary Special template of issue index pattern may cause panic.

### Details

in internal/markup/markup.go

```go link = fmt.Sprintf(`<a href="%s">%s</a>`, com.Expand(metas["format"], metas), m) ```

Issue index pattern is rendered to link with `com.Expand`.

However, `com.Expand` is not safe.

```go i = strings.Index(template, "}") if s, ok := match[template[:i]]; ok { ```

when `{` is found but `}` not found, i comes to 1, template[:-1] will be called, and then panicked


finally, all pages than contains issue index are unavailable.

### PoC

1. set issue index pattern as follow


2. add a commit which point to an issue in its msg


using `#1` above

### Impact

DoS that cause part of pages of the specify repo unavailable.