fix(git): make unsetting upstream idempotent

Saving sync settings with no upstream could cause a fatal Git
error when unsetting upstream on a branch that never had tracking
information. This change guards the operation by checking for an existing
merge configuration before unsetting, making it idempotent. A test was
added to verify that clearing an unconfigured upstream is a no-op.

- Add test ensuring clearing an unconfigured upstream is a no-op

src-tauri/src/git.rs

@@ -836,7 +836,13 @@ pub fn set_branch_upstream(
             )?;
         }
         None => {
-            run_git(&repo, ["branch", "--unset-upstream", branch.as_str()])?;
+            // Saving sync settings with "No upstream" must be idempotent. Git
+            // exits with a fatal error when --unset-upstream is used on a
+            // branch that never had tracking information, which is the normal
+            // state immediately after adding the first remote.
+            if git_config_value(&repo, &format!("branch.{branch}.merge")).is_some() {
+                run_git(&repo, ["branch", "--unset-upstream", branch.as_str()])?;
+            }
         }
     }
     status_for_repo(&repo)
@@ -7661,6 +7667,25 @@ mod tests {
         assert_eq!(remote.upstream, None);
     }
 
+    #[test]
+    fn clearing_an_unconfigured_upstream_is_a_noop() {
+        let repo = init_temp_repo("unset_missing_upstream");
+        commit_initial_file(&repo.path);
+        let branch = git_output_test(&repo.path, ["branch", "--show-current"]);
+        run_git_test(&repo.path, ["remote", "add", "origin", "."]);
+
+        let status = set_branch_upstream(
+            repo.path.to_string_lossy().to_string(),
+            branch.clone(),
+            None,
+        )
+        .expect("saving an empty upstream should not fail");
+
+        assert_eq!(status.current_branch.as_deref(), Some(branch.as_str()));
+        assert_eq!(status.upstream, None);
+        assert!(git_config_value(&repo.path, &format!("branch.{branch}.merge")).is_none());
+    }
+
     #[test]
     fn commit_notes_can_be_created_updated_and_deleted_without_changing_commit() {
         let repo = init_temp_repo("commit_notes_crud");