lib/scanner, vendor: Fix previous commit

Can't do what I did, as the rolling function is not the same as the
non-rolling one. Instead this uses an improved version of the rolling
adler32 to accomplish the same thing. (PR filed on upstream, so should
be able to use that directly in the future.)

vendor/manifest

@@ -46,10 +46,9 @@
 		},
 		{
 			"importpath": "github.com/chmduquesne/rollinghash",
-			"repository": "https://github.com/chmduquesne/rollinghash",
+			"repository": "https://github.com/kastelo/rollinghash",
 			"vcs": "git",
-			"revision": "88b86a92826991b14d01fb43456909fcb8a76b8b",
-			"branch": "master",
+			"branch": "reducealloc",
 			"notests": true
 		},
 		{
@@ -422,4 +421,4 @@
 			"notests": true
 		}
 	]
-}
+}