There are some diff functions that are useful in a rewritten
checkout and this lays some groundwork for that. This contains
three main things:
1. Share the function diff uses to calculate the OID for a file
in the working directory (now named `git_diff__oid_for_file`
2. Add a `git_diff__paired_foreach` function to iterator over
two diff lists concurrently. Convert status to use it.
3. Move all the string/prefix/index entry comparisons into
function pointers inside the `git_diff_list` object so they
can be switched between case sensitive and insensitive
versions. This makes them easier to reuse in various
functions without replicating logic. As part of this, move
a couple of index functions out of diff.c and into index.c.