adds rebase backup slide

basics.html

@@ -821,6 +821,39 @@
         <section id="eof" data-background="img/trex.png">
           <h1>EOF</h1>
         </section>
+
+        <section>
+          <section id="backup" data-markdown>
+            # backup slides
+          </section>
+
+          <section id="collab-conflicts-rebase" data-markdown>
+            ## rebasing
+            ### maintainer
+
+            ```bash
+            git remote add alice https://...
+            git fetch alice
+            git rebase master alice/topic/feature
+            # fix conflicts
+            git push
+            # close merge request
+            ```
+
+            ### contributor
+
+            ```bash
+            git fetch upstream
+            git rebase origin/master topic/feature
+            # fix conflicts
+            # force push because branch history changed
+            git push --force
+            ```
+
+            notes:
+            - manually close merge request because no merge commit
+          </section>
+        </section>
       </div>
     </div>