view: add method to iterate local/remote pairs of certain remote

This is common operation, and we don't need a map of all remote targets
to calculate push action.

lib/src/git.rs

@@ -563,11 +563,10 @@ fn copy_exportable_local_branches_to_remote_view(
 ) {
     let new_local_branches = mut_repo
         .view()
-        .branches()
-        .iter()
-        .filter_map(|(branch, branch_target)| {
-            let old_target = branch_target.remote_targets.get(remote_name).flatten();
-            let new_target = &branch_target.local_target;
+        .local_remote_branches(remote_name)
+        .filter_map(|(branch, targets)| {
+            let old_target = targets.remote_target;
+            let new_target = targets.local_target;
             (!new_target.has_conflict() && old_target != new_target).then_some((branch, new_target))
         })
         .filter(|&(branch, _)| git_ref_filter(&RefName::LocalBranch(branch.to_owned())))

lib/src/refs.rs

@@ -113,6 +113,13 @@ fn find_pair_to_remove(
     None
 }
 
+/// Pair of local and remote targets which usually represents a tracking branch.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct TrackingRefPair<'a> {
+    pub local_target: &'a RefTarget,
+    pub remote_target: &'a RefTarget,
+}
+
 #[derive(Debug, PartialEq, Eq, Clone, Hash)]
 pub struct BranchPushUpdate {
     pub old_target: Option<CommitId>,

lib/src/view.rs

@@ -23,7 +23,7 @@ use crate::backend::CommitId;
 use crate::index::Index;
 use crate::op_store;
 use crate::op_store::{BranchTarget, RefTarget, RefTargetOptionExt as _, WorkspaceId};
-use crate::refs::merge_ref_targets;
+use crate::refs::{merge_ref_targets, TrackingRefPair};
 
 #[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Hash, Debug)]
 pub enum RefName {
@@ -250,6 +250,30 @@ impl View {
         }
     }
 
+    /// Iterates local/remote branch `(name, targets)`s of the specified remote
+    /// in lexicographical order.
+    pub fn local_remote_branches<'a>(
+        &'a self,
+        remote_name: &'a str, // TODO: migrate to per-remote view and remove 'a
+    ) -> impl Iterator<Item = (&'a str, TrackingRefPair<'a>)> + 'a {
+        // TODO: maybe untracked remote_target can be translated to absent, and rename
+        // the method accordingly.
+        self.data
+            .branches
+            .iter()
+            .filter_map(move |(name, branch_target)| {
+                let local_target = &branch_target.local_target;
+                let remote_target = branch_target.remote_targets.get(remote_name).flatten();
+                (local_target.is_present() || remote_target.is_present()).then_some((
+                    name.as_ref(),
+                    TrackingRefPair {
+                        local_target,
+                        remote_target,
+                    },
+                ))
+            })
+    }
+
     pub fn rename_remote(&mut self, old: &str, new: &str) {
         for branch in self.data.branches.values_mut() {
             let target = branch.remote_targets.remove(old).flatten();