Copy version number from previous release state instead of recalculating it

Summary:
When there are no new commits, copy the previous version string instead of recalculating it without increment.  Otherwise if the version format string has date components, then these will change during recalculation even if the increment doesn't.

Now if we run into the logic for copying the build PHID, we copy the version string regardless, which should resolve these issues.

Test Plan: Tested locally as best I could; unfortunately this bug occurs across days.

Reviewers: waynea, #koi, lucasr

Reviewed By: waynea, #koi, lucasr

JIRA Issues: KOI-1154

Differential Revision: https://code.pageuppeople.com/D3850

Risk Rating: 1