diff --git a/knotserver/git/cmd.go b/knotserver/git/cmd.go
--- a/knotserver/git/cmd.go
+++ b/knotserver/git/cmd.go
@@ -40,3 +40,7 @@
 func (g *GitRepo) revParse(extraArgs ...string) ([]byte, error) {
 	return g.runGitCmd("rev-parse", extraArgs...)
 }
+
+func (g *GitRepo) mergeBase(extraArgs ...string) ([]byte, error) {
+	return g.runGitCmd("merge-base", extraArgs...)
+}
diff --git a/knotserver/git/diff.go b/knotserver/git/diff.go
--- a/knotserver/git/diff.go
+++ b/knotserver/git/diff.go
@@ -77,6 +77,16 @@
 	return &nd, nil
 }
 
+func (g *GitRepo) MergeBase(a, b *object.Commit) (*object.Commit, error) {
+	out, err := g.mergeBase(a.Hash.String(), b.Hash.String())
+	if err != nil {
+		return nil, fmt.Errorf("merge-base %s %s: %w", a.Hash, b.Hash, err)
+	}
+
+	hash := plumbing.NewHash(strings.TrimSpace(string(out)))
+	return g.r.CommitObject(hash)
+}
+
 func (g *GitRepo) DiffTree(commit1, commit2 *object.Commit) (*types.DiffTree, error) {
 	tree1, err := commit1.Tree()
 	if err != nil {
diff --git a/knotserver/xrpc/repo_compare.go b/knotserver/xrpc/repo_compare.go
--- a/knotserver/xrpc/repo_compare.go
+++ b/knotserver/xrpc/repo_compare.go
@@ -76,12 +76,17 @@ var combinedPatch []*gitdiff.File
 	var combinedPatchRaw string
 	// we need the combined patch
 	if len(formatPatch) >= 2 {
-		diffTree, err := gr.DiffTree(commit1, commit2)
+		mergeBaseCommit, err := gr.MergeBase(commit1, commit2)
 		if err != nil {
 			x.Logger.Error("error comparing revisions", "msg", err.Error())
 		} else {
-			combinedPatch = diffTree.Diff
-			combinedPatchRaw = diffTree.Patch
+			diffTree, err := gr.DiffTree(mergeBaseCommit, commit2)
+			if err != nil {
+				x.Logger.Error("error comparing revisions", "msg", err.Error())
+			} else {
+				combinedPatch = diffTree.Diff
+				combinedPatchRaw = diffTree.Patch
+			}
 		}
 	}