fix: correct merge-base in PR events from git push of pr/ branch

When pushing a pr/ branch, the ahead slice is reversed by callers before
being passed to generate_patches_or_pr_event_or_pr_updates, making it
oldest-first. The tip/first_commit assignments were backwards (using
first()/last() as if the slice were youngest-first), so the merge-base
was computed as the parent of the PR tip rather than the parent of the
oldest commit. Multi-commit PRs therefore showed only 1 commit when
applied via ngit apply.

Adds an integration test that pushes a two-commit large-file PR branch
and asserts the merge-base tag equals the main branch tip.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/bin/git_remote_nostr/push.rs b/src/bin/git_remote_nostr/push.rs
index bd1188b..870f22d 100644
--- a/src/bin/git_remote_nostr/push.rs
+++ b/src/bin/git_remote_nostr/push.rs
@@ -623,8 +623,8 @@ async fn generate_patches_or_pr_event_or_pr_updates(
     let use_pr = parent_is_pr || git_repo.are_commits_too_big_for_patches(ahead);
 
     if use_pr {
-        let tip = ahead.first().context("no commits")?; // ahead is youngest first
-        let first_commit = ahead.last().context("no commits")?;
+        let tip = ahead.last().context("no commits")?; // ahead is oldest first (callers reverse it)
+        let first_commit = ahead.first().context("no commits")?;
         let push_options_refs: Vec<&str> =
             git_server_push_options.iter().map(String::as_str).collect();
         select_servers_push_refs_and_generate_pr_or_pr_update_event(