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.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a