fix(hooks): skip post-checkout rebuild when PREV_HEAD equals NEW_HEAD (#2421)

git checkout -b <new> reports a branch switch (flag=1) but passes the same SHA
for prev and new HEAD, so the post-checkout hook fired a full changed_paths-less
rebuild — the most expensive graphify op — on essentially every new branch. Add
a POSIX-sh guard that exits 0 when PREV_HEAD == NEW_HEAD; a real branch switch
(PREV != NEW) still rebuilds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

graphify/hooks.py

@@ -355,6 +355,10 @@ if [ "$BRANCH_SWITCH" != "1" ]; then
     exit 0
 fi
 
+# A no-op checkout (e.g. `git checkout -b` with no start point) reports a
+# branch switch but leaves the tree unchanged ΓÇö nothing to rebuild (#2421).
+[ "$PREV_HEAD" = "$NEW_HEAD" ] && exit 0
+
 # Only run if graphify-out/ exists (graph has been built before)
 if [ ! -d "graphify-out" ]; then
     exit 0

tests/test_hooks.py

@@ -614,6 +614,15 @@ def test_hooks_honor_skip_env(name, script):
     )
 
 
+def test_checkout_hook_skips_same_head_noop():
+    """`git checkout -b` with no start point passes identical PREV/NEW heads.
+    Rebuild must short-circuit ΓÇö PREV_HEAD/NEW_HEAD were previously assigned
+    and never read (#2421 / leftover from #1809)."""
+    assert "PREV_HEAD=$1" in _CHECKOUT_SCRIPT
+    assert "NEW_HEAD=$2" in _CHECKOUT_SCRIPT
+    assert '[ "$PREV_HEAD" = "$NEW_HEAD" ] && exit 0' in _CHECKOUT_SCRIPT
+
+
 @pytest.mark.parametrize("name,script", _HOOK_SCRIPTS)
 def test_hooks_skip_linked_worktrees(name, script):
     """Both hooks must short-circuit in a linked worktree (git-dir != common-dir),