Refactor the showsyncstatus function to clean up the code a little bit.
- Don't bother to call populateresponseforphab() if the current commit does not contain a differential revision ID.
- Only call getdiffstatus() once instead of 4 times. getdiffstatus() does cache the result instead of hitting phabricator each time, but it is still better to just re-use the return value ourselves instead of hitting the memoization code.
- Refactor the error handling to avoid having deeply nested conditional blocks.