make appendfile simpler so it does not break with revlogng on windows. it used to cache open files. this made revlogng break because it wants to rename files when splitting .i into .i/.d, but cannot rename or unlink open files on windows. new code is bit slower, but safe on linux and windows. proper fix for too many open/close of changelog/manifest belongs in different place. can get 10% speed improvement back.

abort: repository a not found!
255
requesting all changes
abort: error: Connection refused
255
abort: repository a not found!
255
abort: destination '../a' already exists
255
abort: repository a not found!
255
abort: destination 'q' already exists