Fix failure if base and head branches have the same name (cirrus-acti…

…ons#77)

(cherry picked from commit f9a0642)

entrypoint.sh

@@ -92,10 +92,10 @@ git fetch origin $BASE_BRANCH
 git fetch fork $HEAD_BRANCH
 
 # do the rebase
-git checkout -b $HEAD_BRANCH fork/$HEAD_BRANCH
+git checkout -b fork/$HEAD_BRANCH fork/$HEAD_BRANCH
 if git rebase origin/$BASE_BRANCH; then
   # push back
-  git push --force-with-lease fork $HEAD_BRANCH
+  git push --force-with-lease fork fork/$HEAD_BRANCH:$HEAD_BRANCH
 else
   echo "Automatic rebase failed"
   exit 1