update windows readme to document msys/mingw rxvt problems.

fix util.canonpath on windows. old test for absolute path was not portable. use os.path api instead.

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.