coccinelle: detect struct strbuf passed by value

Passing a struct strbuf by value to a function copies the struct
but shares the underlying character array between caller and callee.
If the callee causes a reallocation, the caller's copy becomes a
dangling pointer, leading to a double-free when strbuf_release() is
called.  There is no coccinelle rule to catch this pattern.

Jeff King suggested adding one during review of the
write_worktree_linking_files() fix [1], and noted that a reporting
rule using coccinelle's Python scripting extensions could emit a
descriptive warning, but we do not currently require Python support
in coccinelle.

Add a transformation rule that rewrites a by-value strbuf parameter
to a pointer.  The detection is identical to what a Python-based
reporting rule would catch; only the presentation differs.  The
resulting diff will not produce compilable code on its own (callers
and the function body still need updating), but the spatch output
alerts the developer that the signature needs attention.  This is
consistent with the other rules in strbuf.cocci, which also rewrite
to the preferred form.

[1] https://lore.kernel.org/git/20260309192600.GC309867@coredump.intra.peff.net/

Signed-off-by: Deveshi Dwivedi <deveshigurgaon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: deveshidwivedi

contrib/coccinelle/strbuf.cocci

@@ -60,3 +60,14 @@ expression E1, E2;
 @@
 - strbuf_addstr(E1, real_path(E2));
 + strbuf_add_real_path(E1, E2);
+
+@@
+identifier fn, param;
+@@
+  fn(...,
+- struct strbuf param
++ struct strbuf *param
+  ,...)
+  {
+  ...
+  }