git: don't detach head if the ref already points to the right place

The first time we export to Git, we don't actually export
anything. That's a little weird and I don't know why I did it that
way. It seems more natural to export the current state. I'd like to
change it to do that. However, that currently means we'll detach the
current HEAD if it points to any of the branches we export. This patch
restructures the code a bit and skips the detach step if the target
branch already points to the right commit.

lib/src/git.rs

@@ -182,14 +182,6 @@ pub fn export_changes(
 ) -> Result<(), GitExportError> {
     let old_branches: HashSet<_> = old_view.branches().keys().cloned().collect();
     let new_branches: HashSet<_> = new_view.branches().keys().cloned().collect();
-    // TODO: Check that the ref is not pointed to by any worktree's HEAD.
-    let mut active_branches = HashSet::new();
-    if let Ok(head_ref) = git_repo.find_reference("HEAD") {
-        if let Some(head_target) = head_ref.symbolic_target() {
-            active_branches.insert(head_target.to_string());
-        }
-    }
-    let mut detach_head = false;
     // First find the changes we want need to make and then make them all at once to
     // reduce the risk of making some changes before we fail.
     let mut refs_to_update = BTreeMap::new();
@@ -216,13 +208,18 @@ pub fn export_changes(
         } else {
             refs_to_delete.insert(git_ref_name.clone());
         }
-        if active_branches.contains(&git_ref_name) {
-            detach_head = true;
-        }
     }
-    if detach_head {
-        if let Ok(head_ref) = git_repo.find_reference("HEAD") {
-            if let Ok(current_git_commit) = head_ref.peel_to_commit() {
+    // TODO: Also check other worktrees' HEAD.
+    if let Ok(head_ref) = git_repo.find_reference("HEAD") {
+        if let (Some(head_target), Ok(current_git_commit)) =
+            (head_ref.symbolic_target(), head_ref.peel_to_commit())
+        {
+            let detach_head = if let Some(new_target) = refs_to_update.get(head_target) {
+                *new_target != current_git_commit.id()
+            } else {
+                refs_to_delete.contains(head_target)
+            };
+            if detach_head {
                 git_repo.set_head_detached(current_git_commit.id())?;
             }
         }