share: store relative share paths with '/' separators
I created a relative share in Windows and tried to use it in WSL, and it failed:
abort: .hg/sharedpath points to nonexistent directory /mnt/c/Users/Matt/hg-review/.hg/..\..\hg\.hg
Use normpath on the read side so that the code has the usual Windows style
paths it always had (I don't think that matters much), but it also eliminates
the directory escaping path components in the case where the path is printed.
This will not fix repositories that have already been created, but it's trivial
enough to hand edit the file to correct it.
Differential Revision: https://phab.mercurial-scm.org/D10330