repoview: protect `base` computation from weird phase root
If for some reason the phase roots contains nullid, the set of filtered revs
will contains -1. That confuse Mercurial a lot. In particular this corrupt the
branchcache.
Standard code path does not result in nullid phase root. It can only result from
altered `.hg/store/phaseroots` or buggy extension. However better safe than
sorry.
try:
import coverage
getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
pass