hash-ll, hashmap: move oidhash() to hash-ll

oidhash() was used by both hashmap and khash, which makes sense.
However, the location of this function in hashmap.[ch] meant that
khash.h had to depend upon hashmap.h, making people unfamiliar with
khash think that it was built upon hashmap.  (Or at least, I personally
was confused for a while about this in the past.)

Move this function to hash-ll, so that khash.h can stop depending upon
hashmap.h.

This has another benefit as well: it allows us to remove hashmap.h's
dependency on hash-ll.h.  While some callers of hashmap.h were making
use of oidhash, most were not, so this change provides another way to
reduce the number of includes.

Diff best viewed with `--color-moved`.

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

hash-ll.h

@@ -270,6 +270,25 @@ static inline void oid_set_algo(struct object_id *oid, const struct git_hash_alg
 	oid->algo = hash_algo_by_ptr(algop);
 }
 
+/*
+ * Converts a cryptographic hash (e.g. SHA-1) into an int-sized hash code
+ * for use in hash tables. Cryptographic hashes are supposed to have
+ * uniform distribution, so in contrast to `memhash()`, this just copies
+ * the first `sizeof(int)` bytes without shuffling any bits. Note that
+ * the results will be different on big-endian and little-endian
+ * platforms, so they should not be stored or transferred over the net.
+ */
+static inline unsigned int oidhash(const struct object_id *oid)
+{
+	/*
+	 * Equivalent to 'return *(unsigned int *)oid->hash;', but safe on
+	 * platforms that don't support unaligned reads.
+	 */
+	unsigned int hash;
+	memcpy(&hash, oid->hash, sizeof(hash));
+	return hash;
+}
+
 const char *empty_tree_oid_hex(void);
 const char *empty_blob_oid_hex(void);