Fix overly inclusive IndexSet::minus_one_range
Fix overly inclusive IndexSet::minus_one_range

Due to a logic error, the minus_one_range method on IndexSet was
including ranges not in the input interval.

Includes test (which should have been more comprehensive).

Fixes #241

modelorganism/xi-editor,modelorganism/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor,google/xi-editor,modelorganism/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor