xdiff: split xrecord_t.ha into line_hash and minimal_perfect_hash

xdiff: split xrecord_t.ha into line_hash and minimal_perfect_hash

The ha field is serving two different purposes, which makes the code
harder to read. At first glance, it looks like many places assume
there could never be hash collisions between lines of the two input
files. In reality, line_hash is used together with xdl_recmatch() to
ensure correct comparisons of lines, even when collisions occur.

To make this clearer, the old ha field has been split:
  * line_hash: a straightforward hash of a line, independent of any
    external context. Its type is uint64_t, as it comes from a fixed
    width hash function.
  * minimal_perfect_hash: Not a new concept, but now a separate
    field. It comes from the classifier's general-purpose hash table,
    which assigns each line a unique and minimal hash across the two
    files. A size_t is used here because it's meant to be used to
    index an array. This also avoids ` as usize` casts on the Rust
    side when using it to index a slice.

Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>