Support de-duping issues (#65)

* De-dupe against open issues when dedupe_issues = true

* Update README

* Fix compile errors

* Add unit tests for issue.ts

src/issue.ts

@@ -19,3 +19,28 @@ export function getIssueOption(body: string): IssueOption {
     labels
   }
 }
+
+export type GetIssuesFunc = (options: {
+  owner: string
+  repo: string
+  state: 'open' | 'closed' | 'all' | undefined
+}) => Promise<{data: {title: string; number: number}[]}>
+
+export async function getExistingIssueNumber(
+  getIssues: GetIssuesFunc,
+  repo: {
+    owner: string
+    repo: string
+  }
+): Promise<number | null> {
+  const {data: issues} = await getIssues({
+    ...repo,
+    state: 'open'
+  })
+
+  const iss = issues
+    .filter(i => i.title === core.getInput('issue_title'))
+    .shift()
+
+  return iss === undefined ? null : iss.number
+}