one of these tests is still failing; there is another edge case left to fix

problem found by jrick ok millert

This fixes spurious 'U' notifications for symlinks during 'got update' that occurred even when the work tree was fully up-to-date. Observed on a work tree of the FreeBSD src repo and reproduced in our test suite by adding a no-op update at the end of a test which deals with updating symlinks.

Condense error checking to reduce a future diff moving those lines around; this pattern is already used in other parts of the tree. fine stsp ok tracey

suggested by mpi@ ages ago