Fix an issue where in some cases 'wrong' commits are shown in listed branches

This was because the merge base computed was wrong. Instead of relying on the default target.sha, which may have become stale, read the git reference of the default target

Author: krlvi

crates/gitbutler-branch-actions/tests/extra/mod.rs

@@ -19,10 +19,12 @@ use gitbutler_branch_actions::{
     BranchManagerExt, Get,
 };
 use gitbutler_commit::{commit_ext::CommitExt, commit_headers::CommitHeadersV2};
+use gitbutler_oxidize::OidExt;
 use gitbutler_reference::{Refname, RemoteRefname};
 use gitbutler_repo::RepositoryExt;
 use gitbutler_stack::{BranchOwnershipClaims, Target, VirtualBranchesHandle};
 use gitbutler_testsupport::{commit_all, virtual_branches::set_test_target, Case, Suite};
+use gix::refs::transaction::PreviousValue;
 use pretty_assertions::assert_eq;
 
 #[test]
@@ -829,6 +831,12 @@ fn merge_vbranch_upstream_clean_rebase() -> Result<()> {
         sha: target_oid,
         push_remote_name: None,
     })?;
+    ctx.gix_repository()?.reference(
+        "refs/remotes/origin/master".to_string(),
+        target_oid.to_gix(),
+        PreviousValue::Any,
+        "",
+    )?;
 
     // add some uncommitted work
     let file_path2 = Path::new("test2.txt");
@@ -959,6 +967,12 @@ fn merge_vbranch_upstream_conflict() -> Result<()> {
         sha: target_oid,
         push_remote_name: None,
     })?;
+    ctx.gix_repository()?.reference(
+        "refs/remotes/origin/master".to_string(),
+        target_oid.to_gix(),
+        PreviousValue::Any,
+        "",
+    )?;
 
     let remote_branch: RemoteRefname = "refs/remotes/origin/g-branch-1".parse().unwrap();
     let branch_manager = ctx.branch_manager();

crates/gitbutler-stack/src/stack.rs

@@ -310,9 +310,19 @@ impl Stack {
     /// - If a target is not set for the project
     /// - If the head commit of the stack is not found
     pub fn merge_base(&self, stack_context: &StackContext) -> Result<git2::Oid> {
-        let target = stack_context.target();
-        let repository = stack_context.repository();
-        let merge_base = repository.merge_base(self.head(&repository.to_gix()?)?, target.sha)?;
+        let gix_repo = stack_context.repository().to_gix()?;
+        let git2_repository = stack_context.repository();
+        let target_sha = gix_repo
+            .find_reference(&stack_context.target().branch.to_string())?
+            .peel_to_commit()?
+            .id;
+        let mut revwalk = git2_repository.revwalk()?;
+        revwalk.hide(self.head(&gix_repo)?)?;
+        revwalk.push(target_sha.to_git2())?;
+        revwalk.simplify_first_parent()?;
+        let last_commit = revwalk.last().ok_or_else(|| anyhow!("fucked"))??;
+        let last_commit = git2_repository.find_commit(last_commit)?;
+        let merge_base = last_commit.parent_id(0)?;
         Ok(merge_base)
     }