[client-app] only upload 7 characters of the commit hash

Summary:
On windows running `git rev-parse --short HEAD` does in fact now give you
9 characters instead of 7 like it does on Mac. This will ensure that
builds get uploaded to the same folder and help ensure we don't post a
version that doesn't exist on the release page.

Test Plan: Manual

Reviewers: juan, halla, spang

Reviewed By: spang

Differential Revision: https://phab.nylas.com/D4217

packages/client-app/build/tasks/upload-task.js

@@ -23,7 +23,7 @@ module.exports = (grunt) => {
         if (error) {
           return reject();
         }
-        const commitHash = stdout ? stdout.trim() : null;
+        const commitHash = (stdout ? stdout.trim() : "").slice(0, 7);
         packageVersion = json.version;
         if (packageVersion.indexOf('-') > 0) {
           fullVersion = packageVersion;