git: when fetching, prune refs that have been deleted on the remote

Otherwise remote-tracking branches just pile up.

It seems that both git and libgit2 remove the remote-tracking branch
when you push a deletion, so `jj branch --delete foo; jj git push
--branch foo` already sees `foo` disappear locally as well. However,
if a branch has been deleted on the remote, we would never know before
this change.

lib/src/git.rs

@@ -14,6 +14,7 @@
 
 use std::collections::BTreeMap;
 
+use git2::FetchPrune;
 use thiserror::Error;
 
 use crate::commit::Commit;
@@ -144,6 +145,7 @@ pub fn fetch(
     });
     let mut fetch_options = git2::FetchOptions::new();
     fetch_options.remote_callbacks(callbacks);
+    fetch_options.prune(FetchPrune::On);
     let refspec: &[&str] = &[];
     remote.fetch(refspec, Some(&mut fetch_options), None)?;
     import_refs(mut_repo, git_repo).map_err(|err| match err {