backend: reduce BackendError size somewhat

One of the error types that I later created embedded `BackendError`, but `clippy` complained that the size of the type was too large. This helps address that.

lib/src/backend.rs

@@ -215,7 +215,9 @@ pub enum BackendError {
     InvalidUtf8 {
         object_type: String,
         hash: String,
-        source: std::string::FromUtf8Error,
+        // Box to reduce size for other error types that include this one:
+        // https://rust-lang.github.io/rust-clippy/master/index.html#result_large_err
+        source: Box<std::string::FromUtf8Error>,
     },
     #[error("Object {hash} of type {object_type} not found: {source}")]
     ObjectNotFound {

lib/src/git_backend.rs

@@ -473,7 +473,7 @@ impl Backend for GitBackend {
             BackendError::InvalidUtf8 {
                 object_type: id.object_type(),
                 hash: id.hex(),
-                source: err,
+                source: Box::new(err),
             }
         })?;
         Ok(target)