Remove --rejoin flags

In newer versions of Git, these arguments cannot be provided due to git/git@9a3e3ca.

Relevant comment:

> It could be argued that this change might break existing users.  I'd argue that those existing users are already broken, and they just don't know it.  Let them know that they're broken.

scripts/rebase

@@ -154,7 +154,7 @@ fi
 # Pull the subdirectory staging branch into the main staging branch
 echo "> Pulling in contents of ${SUB_DIRECTORY_BRANCH} as subtree in ${STAGING_BRANCH} rooted at ${DEST_DIRECTORY}..."
 git checkout ${STAGING_BRANCH} 1>/dev/null 2>/dev/null
-git subtree add -P ${DEST_DIRECTORY} ${SUB_DIRECTORY_BRANCH} --rejoin 1>/dev/null 2>/dev/null
+git subtree add -P ${DEST_DIRECTORY} ${SUB_DIRECTORY_BRANCH} 1>/dev/null 2>/dev/null
 
 # Generate user changes
 PACKAGE=${PACKAGE} make prepare 1>/dev/null 2>/dev/null
@@ -234,7 +234,7 @@ for commit in ${COMMITS}; do
     # Pull in changes from subdirectory branch
     git checkout ${STAGING_BRANCH} 1>/dev/null 2>/dev/null
     set +e
-    git subtree pull -P ${DEST_DIRECTORY} . ${SUB_DIRECTORY_BRANCH} --rejoin 1>/dev/null 2>/dev/null
+    git subtree pull -P ${DEST_DIRECTORY} . ${SUB_DIRECTORY_BRANCH} 1>/dev/null 2>/dev/null
     if [ $? -ne 0 ]; then
         # If the automatic merge has conflicts, reset the staged contents
         git reset HEAD 1>/dev/null 2>/dev/null