Fix regression on push branches

Fixes regression in which IAC will fail to update push branch
when the push branch already exists and checkout branch is
ahead.

The reconciliation errors with: 'cannot push because a reference
that you are trying to update on the remote contains commits
that are not present locally.'

Regression introduced on fluxcd#330.

Signed-off-by: Paulo Gomes <[email protected]>

controllers/imageupdateautomation_controller.go

@@ -735,6 +735,12 @@ var errRemoteBranchMissing = errors.New("remote branch missing")
 // switchToBranch switches to a branch after fetching latest from upstream.
 // If the branch does not exist, it is created using the head as the starting point.
 func switchToBranch(repo *libgit2.Repository, ctx context.Context, branch string, access repoAccess) error {
+	origin, err := repo.Remotes.Lookup(originRemote)
+	if err != nil {
+		return fmt.Errorf("cannot lookup remote: %w", err)
+	}
+	defer origin.Free()
+
 	callbacks := access.remoteCallbacks(ctx)
 	if managed.Enabled() {
 		// Override callbacks with dummy ones as they are not needed within Managed Transport.
@@ -743,6 +749,14 @@ func switchToBranch(repo *libgit2.Repository, ctx context.Context, branch string
 	}
 
 	branchRef := fmt.Sprintf("origin/%s", branch)
+	// Force the fetching of the remote branch.
+	err = origin.Fetch([]string{branch}, &libgit2.FetchOptions{
+		RemoteCallbacks: callbacks,
+	}, "")
+	if err != nil {
+		return fmt.Errorf("cannot fetch remote branch: %w", err)
+	}
+
 	remoteBranch, err := repo.LookupBranch(branchRef, libgit2.BranchRemote)
 	if err != nil && !libgit2.IsErrorCode(err, libgit2.ErrorCodeNotFound) {
 		return err