Fix incremental testing script

Earlier, it would take the first page of commits from the GitHub's PR
commits page.
Now, it follows the next link url to fetch all commits.

Tested with

```
CIRCLE_PROJECT_USERNAME=celo-org CIRCLE_PROJECT_REPONAME=celo-monorepo CIRCLE_PULL_REQUEST="#1152" ./scripts/ci_check_if_test_should_run_v2.sh $PWD/packages/protocol
```

Earlier, this would check (First) 30 commits. Now, it checks all 46
commits.

scripts/check_if_test_should_run_v2.ts

@@ -94,6 +94,12 @@ async function getBranchCommits(): Promise<string[]> {
     }
 
     const url = `https://api.github.com/repos/${owner}/${repo}/pulls/${prNumber}/commits`
+    const commits = await getAllCommits(url)
+    logMessage(`Found ${commits.length} commits in PR ${prNumber}: ${commits}`)
+    return commits
+  }
+
+  async function getAllCommits(url: string): Promise<string[]> {
     const response: any = await fetch(url, {
       headers: {
         Accept: 'application/vnd.github.v3+json',
@@ -114,12 +120,46 @@ async function getBranchCommits(): Promise<string[]> {
       )
     }
     // logMessage(`Commit objects are ${JSON.stringify(commitObjects)}`)
-    const commits = commitObjects.map((commitObject: any) => commitObject.sha)
-    logMessage(`Commits corresponding to ${prNumber} are ${commits}`)
+    let commits = commitObjects.map((commitObject: any) => commitObject.sha)
+
+    const nextPageLink: string | null = getNextPageLink(response)
+    if (nextPageLink) {
+      logMessage(`Getting commits from the next page: ${nextPageLink}`)
+      commits = commits.concat(await getAllCommits(nextPageLink))
+    }
     return commits
   }
 }
 
+// Reference: https://www.w3.org/wiki/LinkHeader
+// linkHeaderValue value would look similar to this
+// <https://api.github.com/repositories/197642503/pulls/1152/commits?page=2>; rel="next", <https://api.github.com/repositories/197642503/pulls/1152/commits?page=2>; rel="last"
+function getNextPageLink(response: Response): string | null {
+  const linkHeaderValue: string | null = response.headers.get('link')
+  if (!linkHeaderValue) {
+    logMessage(`Link header not found in ${response.url}`)
+    return null
+  }
+  logMessage(`Link header value is \"${linkHeaderValue}\"`)
+  const linkEntries = linkHeaderValue.split(',')
+  for (let i = 0; i < linkEntries.length; i++) {
+    if (linkEntries[i].endsWith('rel="next"')) {
+      const lastSemiColon = linkEntries[i].lastIndexOf(';')
+      let nextPageLink = linkEntries[i].substring(0, lastSemiColon)
+      nextPageLink = nextPageLink.trim()
+      if (nextPageLink.startsWith('<')) {
+        nextPageLink = nextPageLink.substring(1)
+      }
+      if (nextPageLink.endsWith('>')) {
+        nextPageLink = nextPageLink.substring(0, nextPageLink.length - 1)
+      }
+      return nextPageLink
+    }
+  }
+  logMessage(`next page link not found in "${linkHeaderValue}"`)
+  return null
+}
+
 async function getChangeCommit(file: string): Promise<string> {
   if (!existsSync(file)) {
     logMessage(`File "${file}" does not exist`)