Add test for blob/tree name collisions in index
index: Compare with given len
index: Implement folder-file checks
More index collision tests
Give index_isrch the same semantics as index_srch

In case insensitive index mode, we would stop at a prefixed entry,
treating the provided search key length as a substring, not the
length of the string to match.
Two-phase index merging

When three-way merging indexes, we previously changed each path
as we read them, which would lead to us adding an index entry for
'foo', then removing an index entry for 'foo/file'.  With the new
index requirements, this is not allowed.  Removing entries in the
merged index, then adding them, resolves this.  In the previous
example, we now remove 'foo/file' before adding 'foo'.