Re-write the blob filename creation loop

This was written in a way which attempted to avoid easily creating an
infinite loop.  But really that's a python idiom and doesn't work very
well in Rust.  Worse, as shown by #2972 it is really easy to still get
this wrong.  Instead do this the rust way, this way the compiler can
also reason properly about the branches and what is unreachable
removing some bogus dead code.

src/blob.rs

@@ -93,9 +93,11 @@ impl<'a> BlobObject<'a> {
         stem: &str,
         ext: &str,
     ) -> Result<(String, fs::File), BlobError> {
-        let max_attempt = 15;
+        const MAX_ATTEMPT: u32 = 16;
+        let mut attempt = 0;
         let mut name = format!("{}{}", stem, ext);
-        for attempt in 1..=max_attempt {
+        loop {
+            attempt += 1;
             let path = dir.join(&name);
             match fs::OpenOptions::new()
                 .create_new(true)
@@ -105,7 +107,7 @@ impl<'a> BlobObject<'a> {
             {
                 Ok(file) => return Ok((name, file)),
                 Err(err) => {
-                    if attempt == max_attempt {
+                    if attempt >= MAX_ATTEMPT {
                         return Err(BlobError::CreateFailure {
                             blobdir: dir.to_path_buf(),
                             blobname: name,
@@ -119,12 +121,6 @@ impl<'a> BlobObject<'a> {
                 }
             }
         }
-        // This is supposed to be unreachable, but the compiler doesn't know.
-        Err(BlobError::CreateFailure {
-            blobdir: dir.to_path_buf(),
-            blobname: name,
-            cause: std::io::Error::new(std::io::ErrorKind::Other, "supposedly unreachable"),
-        })
     }
 
     /// Creates a new blob object with unique name by copying an existing file.