File deduplication (#6332)

When receiving messages, blobs will be deduplicated with the new
function `create_and_deduplicate_from_bytes()`. For sending files, this
adds a new function `set_file_and_deduplicate()` instead of
deduplicating by default.

This is for
https://github.com/deltachat/deltachat-core-rust/issues/6265; read the
issue description there for more details.

TODO:
- [x] Set files as read-only
- [x] Don't do a write when the file is already identical
- [x] The first 32 chars or so of the 64-character hash are enough. I
calculated that if 10b people (i.e. all of humanity) use DC, and each of
them has 200k distinct blob files (I have 4k in my day-to-day account),
and we used 20 chars, then the expected value for the number of name
collisions would be ~0.0002 (and the probability that there is a least
one name collision is lower than that) [^1]. I added 12 more characters
to be on the super safe side, but this wouldn't be necessary and I could
also make it 20 instead of 32.
- Not 100% sure whether that's necessary at all - it would mainly be
necessary if we might hit a length limit on some file systems (the
blobdir is usually sth like
`accounts/2ff9fc096d2f46b6832b24a1ed99c0d6/dc.db-blobs` (53 chars), plus
64 chars for the filename would be 117).
- [x] "touch" the files to prevent them from being deleted
- [x] TODOs in the code

For later PRs:
- Replace `BlobObject::create(…)` with
`BlobObject::create_and_deduplicate(…)` in order to deduplicate
everytime core creates a file
- Modify JsonRPC to deduplicate blob files
- Possibly rename BlobObject.name to BlobObject.file in order to prevent
confusion (because `name` usually means "user-visible-name", not "name
of the file on disk").

[^1]: Calculated with both https://printfn.github.io/fend/ and
https://www.geogebra.org/calculator, both of which came to the same
result
([1](https://github.com/user-attachments/assets/bbb62550-3781-48b5-88b1-ba0e29c28c0d),

[2](https://github.com/user-attachments/assets/82171212-b797-4117-a39f-0e132eac7252))

---------

Co-authored-by: l <link2xt@testrun.org>

src/imex/transfer.rs

@@ -394,7 +394,8 @@ mod tests {
         let file = ctx0.get_blobdir().join("hello.txt");
         fs::write(&file, "i am attachment").await.unwrap();
         let mut msg = Message::new(Viewtype::File);
-        msg.set_file(file.to_str().unwrap(), Some("text/plain"));
+        msg.set_file_and_deduplicate(&ctx0, &file, Some("hello.txt"), Some("text/plain"))
+            .unwrap();
         send_msg(&ctx0, self_chat.id, &mut msg).await.unwrap();
 
         // Prepare to transfer backup.
@@ -428,7 +429,12 @@ mod tests {
         let msg = Message::load_from_db(&ctx1, *msgid).await.unwrap();
 
         let path = msg.get_file(&ctx1).unwrap();
-        assert_eq!(path.with_file_name("hello.txt"), path);
+        assert_eq!(
+            // That's the hash of the file:
+            path.with_file_name("ac1d2d284757656a8d41dc40aae4136"),
+            path
+        );
+        assert_eq!("hello.txt", msg.get_filename().unwrap());
         let text = fs::read_to_string(&path).await.unwrap();
         assert_eq!(text, "i am attachment");