Refactor out a function to determine the SHA for a PR

This functionality is used already for the `/deploy` command, and we are
about to need it also in the `/git-artifacts` and `/release` commands.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

GitForWindowsHelper/issues.js

@@ -52,10 +52,21 @@ const createReactionForIssueComment = async (context, token, owner, repo, commen
     return answer.id
 }
 
+const getPRCommitSHA = async (context, token, owner, repo, pullRequestNumber) => {
+    const answer = await sendGitHubAPIRequest(
+        context,
+        token,
+        'GET',
+        `/repos/${owner}/${repo}/pulls/${pullRequestNumber}`
+    )
+    return answer.head.sha
+}
+
 module.exports = {
     addIssueComment,
     getIssue,
     getIssueComment,
     appendToIssueComment,
-    createReactionForIssueComment
+    createReactionForIssueComment,
+    getPRCommitSHA
 }

GitForWindowsHelper/slash-commands.js

@@ -123,13 +123,8 @@ module.exports = async (context, req) => {
             // The commit hash of the tip commit is sadly not part of the
             // "comment.created" webhook's payload. Therefore, we have to get it
             // "by hand"
-            const githubApiRequest = require('./github-api-request')
-            const { head: { sha: ref } } = await githubApiRequest(
-                console,
-                null,
-                'GET',
-                `/repos/${owner}/${repo}/pulls/${issueNumber}`
-            )
+            const { getPRCommitSHA } = require('./issues')
+            const ref = await getPRCommitSHA(console, await getToken(), owner, repo, issueNumber)
 
             await thumbsUp()