Pushing a merge does not get announced in chat.

**Steps to reproduce:**
1. Add a branch to an existing project.
2. Make some changes in the branch.
3. Add, commit and push the branch's changes.
4. Checkout master.
5. Merge new branch with master.
6. Push the new branch to origin/master.

**Expected behavior:**
The push will be announced in the team's general chat like any other push.

**Actual behavior:**
From the Keybase Git UI, the "last push" shows the updated push time and "Announce pushes in #general" is selected, but the push is not announced in the team's general chat.

**Versions:**
Keybase GUI Version: 1.0.45-20180305115557+3a1c65a (Windows)

No files, tests, or entry points are named. Start by reproducing the merge-push flow in the Keybase Git UI with push announcements enabled, then trace how the Windows client sends push events to the team's general chat. Done means merge pushes are announced there like other pushes, with regression coverage added if the relevant test area is found.

Written by the indexing model from the issue text.