Messages not silenced when interrogating source caches

This is a regression from centralizing state handling in the Element class.

The following status messages are extra verbose and intended to be silenced by default:

[--:--:--][        ][ main:core-deps/gstreamer.bst-0     ] STATUS  Running host command

    /usr/bin/git cat-file -t a1bf0f0e6a8118eab309707f52a64e1ce2c2be95

[--:--:--][        ][ main:core-deps/gstreamer.bst-0     ] STATUS  Running host command

    /usr/bin/git show a1bf0f0e6a8118eab309707f52a64e1ce2c2be95:.gitmodules

[--:--:--][        ][ main:core-deps/gstreamer.bst-0     ] STATUS  Running host command

    /usr/bin/git ls-tree a1bf0f0e6a8118eab309707f52a64e1ce2c2be95 common

Since we centralized these source cache state interrogations which we run in the main process, the messages are no longer silenced by default.

We probably need a context manager for silencing the less important messages (as we have an option for in the timed_activity context managers), so we can do this generically from element.py when updating the state of an element in the main process.