This script uses the list of heads and their origin multi-head "git
fetch" left in the $GIT_DIR/FETCH_HEAD file, and makes an octopus
merge on top of the current HEAD using them.
The implementation tries to be strict for the sake of safety. It
insists that your working tree is clean (no local changes) and matches
the HEAD, and when any of the merged heads does not automerge, the
whole process is aborted and tries to rewind your working tree is to
the original state.