AppendRedline() has a boolen return value which is rather
unclear and confusing: most callers don't even check it, but
SaveMergeRedline::InsertRedline() expects "true" to mean that
its redline hasn't been deleted, whereas makeRedline()
expects "true" to mean that the redline was somehow "valid",
even if it has been deleted and merged with an existing one.

The "bMerged" variable in AppendRedline(), which is the source
of the confusion, was introduced with commit
81286906 "docx import fixes
for: redlines".

Split these differing expectations into different return
values of a new enum type.

Change-Id: If81631bde49ee52a249f5ba1dd64ab8e92fffaf7