Do not expand at all during diff
Always shrink and never expand keywords during a diff operation.
Avoid user distraction e.g. because of spurious differences
appearing in the commit editor.
Restore restricted read mode value after overwriting
Even though just enforcing expansion after overwriting files in
the working directory caused no problems that we know of, this avoids
a potential source of problems (e.g. in collaboration other extensions)
at no costs.