rustfmt.toml

windows: continue looking at `%HOME%` for user config files with py3.8+

The `%HOME%` variable is explicitly called out in `hg help config` as a location
that is consulted when reading user files, but python stopped looking at it
when expanding '~' in py3.8+.[1]  Restore that old functionality by copying in
the old implementation (and simplifying it to just use bytes).  It could be
simplfied further, since only '~' is passed, but I'm not sure yet if we need to
make this a generic utility function on Windows.  There are other uses of
`os.path.expanduser()`, but this is the only case I know of that documents
`%HOME%` usage.

(The reason for removing it was that it typically isn't set, but it actually is
set in MSYS and PowerShell, and `%HOME%` and `%USERPROFILE%` are different in
MSYS.  I could be convinced to just replace all uses with this as a general
utility, so we don't have to think too hard about BC.)

[1] https://bugs.python.org/issue36264

Differential Revision: https://phab.mercurial-scm.org/D9559

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