Check HMAC in smudge and diff commands

Git-crypt's position has always been that authentication is best left
to Git, since 1) Git provides immutable history based on SHA-1 hashes
as well as GPG-signed commits and tags, and 2) git-crypt can't be used
safely anyways unless the overall integrity of your repository is assured.

But, since git-crypt already has easy access to a (truncated) HMAC of the
file when decrypting, there's really no reason why git-crypt shouldn't
just verify it and provide an additional layer of protection.

util.hpp

@@ -71,6 +71,7 @@ void		store_be32 (unsigned char*, uint32_t);
 bool		read_be32 (std::istream& in, uint32_t&);
 void		write_be32 (std::ostream& out, uint32_t);
 void*		explicit_memset (void* s, int c, size_t n);	// memset that won't be optimized away
+bool		leakless_equals (const void* a, const void* b, size_t len); // compare bytes w/o leaking timing
 void		init_std_streams ();
 mode_t		util_umask (mode_t);
 int		util_rename (const char*, const char*);