Fix small breakage caused by gc
Fix small breakage caused by gc

The compute_undo function assumed that the first revision was an undo
rather than an edit, which used to be true before gc, but can't be
relied on after. This commit change compute_undo not to assume that.

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