git_store: wait 1 minute for lock on refs to help tests

`test_commit_parallel` was failing on Mac in the GitHub CI. I suspect
the reason was that it was timing out. The test runs in about 1 s on
my Linux desktop and in about 3 s on my Mac laptop. It failed after 31
in the GitHub CI. This patch increases the timeout to 1 minute to try
to make the test pass. It would be better to set the timeout to a
higher value only in tests, but this will be good enough for now. By
the way, it has turned out that git notes (at least libgit2's
implementation of them) are too slow, so we should probably eventually
create our own storage for the extra metadata instead.

lib/src/git_store.rs

@@ -146,7 +146,7 @@ fn write_note(
     };
     let mut backoff = ExponentialBackoff {
         initial_interval: Duration::from_millis(1),
-        max_elapsed_time: Some(Duration::from_secs(10)),
+        max_elapsed_time: Some(Duration::from_secs(60)),
         ..Default::default()
     };
     try_write_note