windows: don't consider '$$' to be an escaped '$' when translating to cmd.exe
This functionality was inherited from `os.path.expandvars()`. But the point of
adding this translating code is to be able to write a portable hook, and bash
wouldn't replace '$$' with '$'. Escaping with '\' works, and is portable.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..