Check calcHash() for errors when creating buffer

Even though we only call calcHash() if `size > LargeFileThreshold` is
false, it may still return ErrFileTooLarge if `size` is smaller than
the actual size of the file that we've just read from disk, i.e. if the
file on disk has increased since we checked it via os.Stat().

internal/buffer/buffer.go

@@ -416,7 +416,10 @@ func NewBuffer(r io.Reader, size int64, path string, startcursor Loc, btype BufT
 		} else if !hasBackup {
 			// since applying a backup does not save the applied backup to disk, we should
 			// not calculate the original hash based on the backup data
-			calcHash(b, &b.origHash)
+			err := calcHash(b, &b.origHash)
+			if err == ErrFileTooLarge {
+				b.Settings["fastdirty"] = true
+			}
 		}
 	}