try to fix regression on Windows due to tempfiles semantics (#3601)

Until `perist()` is truly atomic also on Windows, and modelled
[after Git](https://github.com/git/git/blob/master/compat/mingw.c#L2159-L2209),
sans the retries, it seems to work to just perform ordinary writes.

Before going there though, we try to call `sync_all()` on Windows to truly
finish the operation. On Unix, a rename is already supposed to be atomic.

crates/gitbutler-core/src/fs.rs

@@ -87,7 +87,12 @@ fn persist_tempfile(
     to_path: impl AsRef<Path>,
 ) -> std::io::Result<()> {
     match tempfile.persist(to_path) {
-        Ok(Some(_opened_file)) => Ok(()),
+        Ok(Some(_opened_file)) => {
+            // EXPERIMENT: Does this fix #3601?
+            #[cfg(windows)]
+            _opened_file.sync_all()?;
+            Ok(())
+        }
         Ok(None) => unreachable!(
             "BUG: a signal has caused the tempfile to be removed, but we didn't install a handler"
         ),