Only read .hg/hgrc files from trusted users/groups
The list of trusted users and groups is specified in the [trusted]
section of a hgrc; the current user is always trusted; "*" can be
used to trust all users/groups.
Global hgrc files are always read.
On Windows (and other systems that don't have the pwd and grp modules),
all .hg/hgrc files are read.
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'