Fix #21740.
Updated the Pull Request template so that the 'Update branch by merge'
button is visible for WIP PR's. Making the behaviour match a non WIP-PR.
Previous WIP page with changes pending on the branch:

Updated UI adding the update button:

## Notes
* have not removed the **$canAutoMerge** variable from the pull.tmpl on
this
[line](
|
||
|---|---|---|
| .. | ||
| admin | ||
| api/packages/pypi | ||
| base | ||
| code | ||
| custom | ||
| explore | ||
| org | ||
| package | ||
| projects | ||
| repo | ||
| shared | ||
| status | ||
| swagger | ||
| user | ||
| home.tmpl | ||
| install.tmpl | ||
| post-install.tmpl | ||