Compare tree hashes instead of using git diff

Comparing tree hashes is more accurate for detecting content differences
between the merge commit and PR head, avoiding false positives when
the content is identical but commit SHAs differ.

Author: IanButterworth

test_harness.jl

@@ -25,8 +25,10 @@ if haskey(ENV, "GITHUB_SHA") && get(ENV, "GITHUB_EVENT_NAME", "") == "pull_reque
 
         # Check if there's any difference between the merge commit and the PR head
         # In GitHub Actions, HEAD^2 is the PR head (second parent of merge commit)
-        # success() returns true if the command exits with 0 (no differences)
-        has_diff = !success(`git diff --quiet --exit-code HEAD^2 HEAD`)
+        # Compare tree hashes to check if content actually differs
+        merge_tree = chomp(read(`git rev-parse HEAD^{tree}`, String))
+        pr_tree = chomp(read(`git rev-parse HEAD^2^{tree}`, String))
+        has_diff = merge_tree != pr_tree
 
         if has_diff
             base_branch = isempty(base_branch_name) ? "the base branch" : "'$base_branch_name'"