use MatchingKey when a Key is known

use MatchingKey when a Key is known

This fixes a bug where a file that was not preferred content could be
transferred to a remote. This happened when the file got deleted after
the sync started running.

The only time checkMatcher is run without a Key is in calls to
checkFileMatcher, which are only done by add, addurl, import, and
smudge --clean. Those won't be affected by this kind of race. Anything
else that might be precaching and have a similar race as sync will also
be fixed, but I don't know if it actually affected anything other than
sync.

As well as fixing a bug, this also probably makes sync and --auto faster
by avoiding the redundant key lookup.

This commit was sponsored by Graham Spencer on Patreon.