Bug #1722: Update release notes script according to new submodule practices

Currently the script is used like this:

if $(git describe > /dev/null 2>&1); then
  # brief notes
  NOTES="$TARGET_BUILD_DIR/$DISTRIBUTION_FOLDER release notes.txt"
  "$BUILDTOOLS_DIR/ReleaseNotes.sh" > "$NOTES"
  echo -n "WOPublic: " >> "$NOTES"
  (cd "$SOURCE_ROOT/WOPublic" && "$BUILDTOOLS_DIR/ReleaseNotes.sh" --tag-prefix=Synergy- >> "$NOTES")
  echo -n "buildtools: " >> "$NOTES"
  (cd "$SOURCE_ROOT/buildtools" && "$BUILDTOOLS_DIR/ReleaseNotes.sh" --tag-prefix=Synergy- >> "$NOTES")

  # detailed notes
  NOTES="$TARGET_BUILD_DIR/$DISTRIBUTION_FOLDER release notes (detailed).txt"
  "$BUILDTOOLS_DIR/ReleaseNotes.sh" --long > "$NOTES"
  echo -n "WOPublic: " >> "$NOTES"
  (cd "$SOURCE_ROOT/WOPublic" && "$BUILDTOOLS_DIR/ReleaseNotes.sh" --long --tag-prefix=Synergy- >> "$NOTES")
  echo -n "buildtools: " >> "$NOTES"
  (cd "$SOURCE_ROOT/buildtools" && "$BUILDTOOLS_DIR/ReleaseNotes.sh" --long --tag-prefix=Synergy- >> "$NOTES")
fi

As you can see, we produce the notes for the parent project (in this case, Synergy) and then append notes for the dependent projects (actually submodules).

This worked fine with the old policy of tagging the dependent projects, but it falls down now that we are no longer doing such tagging. (The tagging dates back to when the projects were truly independent, but we have since switched to using submodules, which means that such explicit tagging is no longer required to identify which version of the dependencies was used in a given release.)

So the release notes script needs to be updated to:

1. Produce release notes as before in the host project, based on tags
2. Produce release notes for each submodule, not based on tags inside those submodules but rather based on the starting and ending points indicated by the submodule references in the superproject.