rerere: use strmap to store rerere directories

We store a struct for each directory we access under .git/rr-cache. The
structs are kept in an array sorted by the binary hash associated with
their name (and we do lookups with a binary search).

This works OK, but there are a few small downsides:

 - the amount of code isn't huge, but it's more than we'd need using one
   of our other stock data structures

 - the insertion into a sorted array is quadratic (though in practice
   it's unlikely anybody has enough conflicts for this to matter)

 - it's intimately tied to the representation of an object hash. This
   isn't a big deal, as the conflict ids we generate use the same hash,
   but it produces a few awkward bits (e.g., we are the only user of
   hash_pos() that is not using object_id).

Let's instead just treat the directory names as strings, and store them
in a strmap. This is less code, and removes the use of hash_pos().

Insertion is now non-quadratic, though we probably use a bit more
memory. Besides the hash table overhead, and storing hex bytes instead
of a binary hash, we actually store each name twice. Other code expects
to access the name of a rerere_dir struct from the struct itself, so we
need a copy there. But strmap keeps its own copy of the name, as well.

Using a bare hashmap instead of strmap means we could use the name for
both, but at the cost of extra code (e.g., our own comparison function).
Likewise, strmap has a feature to use a pointer to the in-struct name at
the cost of a little extra code. I didn't do either here, as simple code
seemed more important than squeezing out a few bytes of efficiency.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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