cache-tree: trace regions for I/O

cache-tree: trace regions for I/O

As we write or read the cache tree index extension, it can be good to
isolate how much of the file I/O time is spent constructing this
in-memory tree from the existing index or writing it out again to the
new index file. Use trace2 regions to indicate that we are spending time
on this operation.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>