Since the overview stuff is a synchronization point anyways,
move it into the main V2Writable process and allow us to
drop a bunch of code. This is another step towards making
Xapian optional for v2.
In other words, the fan-out point is moved and the Xapian
partitions no longer need to synchronize against each other:
Before:
/-------->\
/---------->\
v2writable -->+----parts----> over
\---------->/
\-------->/
After:
/---------->
/----------->
v2writable --> over-->+----parts--->
\----------->
\---------->
Since the overview/threading logic needs to run on the same core
that feeds git-fast-import, it's slower for small repos but is
not noticeable in large imports where I/O wait in the partitions
dominates.