git.rs: create export_some_refs

lib/src/git.rs

@@ -325,6 +325,14 @@ pub enum GitExportError {
 pub fn export_refs(
     mut_repo: &mut MutableRepo,
     git_repo: &git2::Repository,
+) -> Result<Vec<String>, GitExportError> {
+    export_some_refs(mut_repo, git_repo, |_| true)
+}
+
+pub fn export_some_refs(
+    mut_repo: &mut MutableRepo,
+    git_repo: &git2::Repository,
+    git_ref_filter: impl Fn(&RefName) -> bool,
 ) -> Result<Vec<String>, GitExportError> {
     // First find the changes we want need to make without modifying mut_repo
     let mut branches_to_update = BTreeMap::new();
@@ -347,6 +355,7 @@ pub fn export_refs(
             // We *do* need to worry about exporting deletion of remote-tracking branches by `jj
             // branch forget`.
         )
+        .filter(git_ref_filter)
         .collect();
     for jj_known_ref in jj_known_refs {
         let (nice_branch_name, new_branch) = match &jj_known_ref {