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

This can make a huge difference on repositories which contain more than 16 pack files, at the expense of additional open file handles. My test case was a ports.git repository with > 60 pack files (these accumulate due to 'got fetch', and 'git repack -a' would condense them). With cache size 16 'got update' in /usr/ports took more than 60 seconds. With cache size 64 the same command took less than 5 seconds. The root cause is probably that mapping and unmapping large files over and over again is expensive on OpenBSD.

found by naddy ok naddy jrick