Fix logic in get_compressed_holdings()

Fix logic in get_compressed_holdings()

[This commit has been squashed for merging. LFW]

  * This commit rearranges some of the logic branches to protect
    against an unusual case of having two holding statements with
    the same start value, but one being open-ended and one not.

  * The logic in get_combined_holdings() was a little sloppy and
    repeated some steps unnecessarily.  This cleans things up.

    See the test case in the previous commit for more clarity.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>