bloom.c: add the murmur3 hash implementation

In preparation for computing changed paths Bloom filters,
implement the Murmur3 hash algorithm as described in [1].
It hashes the given data using the given seed and produces
a uniformly distributed hash value.

[1] https://en.wikipedia.org/wiki/MurmurHash#Algorithm

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Helped-by: Szeder Gábor <szeder.dev@gmail.com>
Reviewed-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bloom.h

@@ -0,0 +1,13 @@
+#ifndef BLOOM_H
+#define BLOOM_H
+
+/*
+ * Calculate the murmur3 32-bit hash value for the given data
+ * using the given seed.
+ * Produces a uniformly distributed hash value.
+ * Not considered to be cryptographically secure.
+ * Implemented as described in https://en.wikipedia.org/wiki/MurmurHash#Algorithm
+ */
+uint32_t murmur3_seeded(uint32_t seed, const char *data, size_t len);
+
+#endif