fileset: detect unintentional existing() invocation at runtime A fileset predicate can invoke 'matchctx.existing()' successfully, even if it isn't marked as "existing caller". It is aborted only in some corner cases: e.g. there were one deleted file in the working directory (see 8a0513bf030a for detail). This patch makes 'matchctx.existing()' invocation abort if not '_existingenabled', which is true only while "existing caller" running. After this changes, non-"existing caller" predicate function is aborted immediately, whenever it invokes 'matchctx.existing()'. This prevent developer from forgetting to mark a predicate as "existing caller". BTW, unintentional 'matchctx.status()' invocation can be detected easily without any additional trick like this patch, because it returns 'None' if a predicate isn't marked as "status caller", and referring field (e.g. '.modified') of it is always aborted.