branchmap: `stream_in` write remote branchcache to local one
The previous code was writing it to a non existent `branchcache` attribute. We
now write is to the proper `_branchcache` attribute and initialize the
`_branchcachetip` at the same time.
We keep writing it to disk, the previous code had this part right.
try:
import coverage
getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
pass