Add instructions to rebase and push branch before merging

Author: yalsayyad

work/flow.rst

@@ -474,11 +474,22 @@ We **always use non-fast forward merges** so that the merge point is marked in G
 
 .. code-block:: bash
 
+   # Prepare to merge
+   git checkout master
+   git pull  # Sanity check; if master is up-to-date, skip ahead to "Merge" below
+   git checkout tickets/DM-NNNN
+   git rebase master
+   git push --force  # so that the commits on the remote branch match the commits merged*
+   # Merge
    git checkout master
-   git pull  # Sanity check; rebase ticket if master was updated.
    git merge --no-ff tickets/DM-NNNN
    git push
 
+We force push the rebased branch for three reasons:
+(1) In many repos, branch protection requires that Travis was run on any commits before they can be pushed onto master
+(2) The policy is to delete branches that have been merged. This is only possible if the exact commit has been merged.
+(3) For convenience, Github will automatically close pull requests if the corresponding branch has been merged to master.
+
 **GitHub pull request pages also offer a 'big green button' for merging a branch to master**.
 We discourage you from using this button since there isn't a convenient way of knowing that the merged development history graph will be linear from GitHub's interface.
 Rebasing the ticket branch against ``master`` and doing the non-fast forward merging on the command line is the safest workflow.