seek: defer matcher check until more info is known

Sped up seeking for files to operate on, when using options like --copies
or --in, by around 20%.

Benchmark showed an increase for --copies from 155 seconds to 121
seconds, and --in remote will be similar to that.

For --in here, the speedup was less, 5-10% or so.

(both warm cache)

This commit was sponsored by Jack Hill on Patreon.

doc/todo/faster_key_lookup_for_limits.mdwn

@@ -11,3 +11,5 @@ log for limits that need that), otherwise before getting the key.
 > So this needs a way to introspect a limit to see if the terms used in it
 > match some criteria. Another todo that also needs that is
 > [[sync_fast_import]] --[[Joey]]
+
+[[done]] --[[Joey]]