chunks cache sync log output

[ThomasWaldmann]

$ borg-1.0.11 create -v tw@localhost:repo::test input/
Synchronizing chunks cache...
Archives: 9, w/ cached Idx: 0, w/ outdated Idx: 0, w/o cached Idx: 9.
Fetching and building archive index for test2 ...
Merging into master chunks index ...
...
Fetching and building archive index for test5 ...
Merging into master chunks index ...
Done.

In #2866 the question came up whether that is too verbose and should not be shown without --progress given.

1.1.0 output is the same.