Fix whitespace in commit message, restores part of the test cases

Author: helmo

git-subtree.sh

@@ -367,7 +367,7 @@ rejoin_msg()
         commit_message="Split '$dir/' into commit '$latest_new'"
     fi
     cat <<-EOF
-    $commit_message
+$commit_message
 
     git-subtree-dir: $dir
     git-subtree-mainline: $latest_old