test(hooks): replace static string-match with a runtime same-head test (#2421)

The PR's test only asserted the guard line exists in the source — the exact
static-assertion anti-pattern #2126/#2641 removed for providing zero coverage.
Replace it with a runtime test that runs the real emitted post-checkout script
under sh up to the guard (with a sentinel, before the launch): same-head skips,
different-head falls through, file-checkout skips at the earlier guard. Adds the
CHANGELOG entry.

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

CHANGELOG.md

@@ -4,6 +4,7 @@ Full release notes with details on each version: [GitHub Releases](https://githu
 
 ## 0.9.46 (unreleased)
 
+- Fix: the `post-checkout` hook skips its rebuild when HEAD is unchanged (e.g. `git checkout -b` with no start point), so creating a branch no longer triggers a full graph rebuild (#2421, thanks @nothariharan).
 - Feature: Markdown nodes now carry a `node_kind` (`page` vs `heading`) attribute so a docs corpus can be filtered by kind, and leading YAML frontmatter is parsed onto the page node as bounded, sanitized attributes; a `#` comment inside frontmatter is no longer mis-extracted as a heading (thanks @evanthomasgelders). Node ids are unchanged, so existing markdown graphs are not re-keyed.
 
 - Feature: Common Lisp `.lisp`/`.cl`/`.lsp`/`.asd` extraction via tree-sitter-commonlisp (optional `[commonlisp]` extra) — packages, classes, functions, methods, generics, macros, variable definers, and same-file calls; `open`ed/`:use`d packages resolve cross-file (thanks @fade).

tests/test_hooks.py

@@ -614,13 +614,30 @@ 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" = "$NEW_HEAD" ] && exit 0' in _CHECKOUT_SCRIPT
+def test_checkout_hook_skips_same_head_noop_at_runtime():
+    """`git checkout -b` with no start point reports a branch switch (flag=1) but
+    passes identical PREV/NEW heads, so the rebuild must short-circuit (#2421).
+    Prove BEHAVIOR by running the real emitted script under sh up to the guard
+    with a sentinel, not by matching the source string (per the #2126/#2641
+    convention that static assertions provided zero coverage)."""
+    from graphify.hooks import _CHECKOUT_SCRIPT
+    guard = '[ "$PREV_HEAD" = "$NEW_HEAD" ] && exit 0'
+    assert guard in _CHECKOUT_SCRIPT, "guard missing from the checkout script"
+    # Real script through the same-head guard, then a sentinel — stops before the
+    # graphify-out check / detached launch so nothing is actually rebuilt.
+    prefix = _CHECKOUT_SCRIPT.split(guard)[0] + guard + "\necho RAN\n"
+
+    def run(prev, new, flag):
+        # sh -c CMD name arg1 arg2 arg3  ->  $0=name $1=prev $2=new $3=flag
+        return subprocess.run(["sh", "-c", prefix, "hook", prev, new, flag],
+                              capture_output=True, text=True)
+
+    # branch switch (flag=1), SAME head -> short-circuit, sentinel not reached
+    assert "RAN" not in run("abc123", "abc123", "1").stdout
+    # branch switch (flag=1), DIFFERENT head -> falls through to the sentinel
+    assert "RAN" in run("abc123", "def456", "1").stdout
+    # file checkout (flag != 1) -> skips at the earlier BRANCH_SWITCH guard
+    assert "RAN" not in run("abc123", "def456", "0").stdout
 
 
 @pytest.mark.parametrize("name,script", _HOOK_SCRIPTS)