util: add removedirs as platform depending function According to fa901423ac23 introducing "windows._removedirs()": If a hg repository including working directory is a reparse point (directory symlinked or a junction point), then using os.removedirs will remove the reparse point erroneously. "windows._removedirs()" should be used instead of "os.removedirs()" on Windows. This patch adds "removedirs" as platform depending function to replace "os.removedirs()" invocations for portability and safety