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](
|
||
|---|---|---|
| .. | ||
| add_reaction.tmpl | ||
| attachments.tmpl | ||
| comments.tmpl | ||
| comments_delete_time.tmpl | ||
| context_menu.tmpl | ||
| pull.tmpl | ||
| pull_merge_instruction.tmpl | ||
| reactions.tmpl | ||
| reference_issue_dialog.tmpl | ||
| sidebar.tmpl | ||
| update_branch_by_merge.tmpl | ||