Allocate space for the file before we write it

Not sure if this is a good idea in the end or not, it harms debugging
but creates cleaner files, which on SSDs probably does not matter but
on spinning disks it can. But we use SSDs, so I am totally guilty of
optimizing prematurely here.

src/main.rs

@@ -314,7 +314,20 @@ impl FileReceiver {
                 if let Some(dir) = path.parent() {
                     std::fs::create_dir_all(dir)?;
                 }
-                File::create(path)?
+                let file = File::create(path)?;
+
+                // Resize the file to its final size already:
+                // * So that the file system can do a better job of allocating
+                //   a single extent for it, and it doesn't have to fragment
+                //   the file.
+                // * If we run out of space, we learn about that before we waste
+                //   time on the transfer (although then maybe we should do it
+                //   before we receive a chunk after all?).
+                // This can make debugging a bit harder, because when you look
+                // at just the file size you might think it's fully transferred.
+                file.set_len(self.total_len)?;
+
+                file
             }
             Some(f) => f,
         };