The config and attrcache file reading code would attempt to load a file
in a home directory by expanding the `~` and looking for the file, using
`git_sysdir_find_global_file`.  If the file was not found, the error
handling would look for the literal path, eg `~/filename.txt`.

Use the new `git_config_expand_global_file` instead, which allows us to
get the path to the file separately, when the path is prefixed with
`~/`, and fail with a not found error without falling back to looking
for the literal path.