read-cache: split out function to drop unmerged entries to stage 0

read-cache: split out function to drop unmerged entries to stage 0

In `repo_read_index_unmerged()` we read the index and then drop any
unmerged entries to stage 0. In a subsequent commit we'll want to
perform this operation on arbitrary indexes, not only the one of the
given repository.

Prepare for this by splitting out the functionality into a new function
that can act on an arbitrary index.

While at it, fix a signedness mismatch when iterating through the index
cache entries.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>