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.
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3