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