repository/master_index: Optimize Index.Lookup()

When looking up a blob in the master index, with several
indexes present in the master index, a significant amount of time
is spent generating errors for each failed lookup.  However, these
errors are often used to check if a blob is present, but the contents
are not inspected making the overhead of the error not useful.

Instead, change Index.Lookup (and Index.LookupSize) to instead return
a boolean denoting if the blob was found instead of an error.  Also change
all the calls to these functions to handle the new function signature.

benchmark                                            old ns/op     new ns/op     delta
BenchmarkMasterIndexLookupSingleIndex-6              820           897           +9.39%
BenchmarkMasterIndexLookupMultipleIndex-6            12821         2001          -84.39%
BenchmarkMasterIndexLookupSingleIndexUnknown-6       5378          492           -90.85%
BenchmarkMasterIndexLookupMultipleIndexUnknown-6     17026         1649          -90.31%

benchmark                                            old allocs     new allocs     delta
BenchmarkMasterIndexLookupSingleIndex-6              9              9              +0.00%
BenchmarkMasterIndexLookupMultipleIndex-6            59             19             -67.80%
BenchmarkMasterIndexLookupSingleIndexUnknown-6       22             6              -72.73%
BenchmarkMasterIndexLookupMultipleIndexUnknown-6     72             16             -77.78%

benchmark                                            old bytes     new bytes     delta
BenchmarkMasterIndexLookupSingleIndex-6              160           160           +0.00%
BenchmarkMasterIndexLookupMultipleIndex-6            3200          240           -92.50%
BenchmarkMasterIndexLookupSingleIndexUnknown-6       1232          48            -96.10%
BenchmarkMasterIndexLookupMultipleIndexUnknown-6     4272          128           -97.00%

internal/fuse/file_test.go

@@ -81,8 +81,8 @@ func TestFuseFile(t *testing.T) {
 		memfile  []byte
 	)
 	for _, id := range content {
-		size, err := repo.LookupBlobSize(id, restic.DataBlob)
-		rtest.OK(t, err)
+		size, found := repo.LookupBlobSize(id, restic.DataBlob)
+		rtest.Assert(t, found, "Expected to find blob id %v", id)
 		filesize += uint64(size)
 
 		buf := restic.NewBlobBuffer(int(size))