typing: add type annotations to the `mercurial.util.filestat` class
It's referenced in the `vfs` classes, so get this out of the way to help there.
The `TypeVar` definition and its usage was copied from the existing `util.pyi`
file.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
end_of_line = lf
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
end_of_line = lf
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false
end_of_line = lf