coccinelle: avoid wrong transformation suggestions from commit.cocci

The semantic patch 'contrib/coccinelle/commit.cocci' added in
2e27bd7731 (treewide: replace maybe_tree with accessor methods,
2018-04-06) is supposed to "ensure that all references to the
'maybe_tree' member of struct commit are either mutations or accesses
through get_commit_tree()".  So get_commit_tree() clearly must be able
to directly access the 'maybe_tree' member, and 'commit.cocci' has a
bit of a roundabout workaround to ensure that get_commit_tree()'s
direct access in its return statement is not transformed: after all
references to 'maybe_tree' have been transformed to a call to
get_commit_tree(), including the reference in get_commit_tree()
itself, the last rule transforms back a 'return get_commit_tree()'
statement, back then found only in get_commit_tree() itself, to a
direct access.

Unfortunately, already the very next commit shows that this workaround
is insufficient: 7b8a21dba1 (commit-graph: lazy-load trees for
commits, 2018-04-06) extends get_commit_tree() with a condition
directly accessing the 'maybe_tree' member, and Coccinelle with
'commit.cocci' promptly detects it and suggests a transformation to
avoid it.  This transformation is clearly wrong, because calling
get_commit_tree() to access 'maybe_tree' _in_ get_commit_tree() would
obviously lead to recursion.  Furthermore, the same commit added
another, more specialized getter function get_commit_tree_in_graph(),
whose legitimate direct access to 'maybe_tree' triggers a similar
wrong transformation suggestion.

Exclude both of these getter functions from the general rule in
'commit.cocci' that matches their direct accesses to 'maybe_tree'.
Also exclude load_tree_for_commit(), which, as static helper funcion
of get_commit_tree_in_graph(), has legitimate direct access to
'maybe_tree' as well.

The last rule transforming back 'return get_commit_tree()' statements
to direct accesses thus became unnecessary, remove it.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/commit.cocci

@@ -10,11 +10,15 @@ expression c;
 - c->maybe_tree->object.oid.hash
 + get_commit_tree_oid(c)->hash
 
+// These excluded functions must access c->maybe_tree direcly.
 @@
+identifier f !~ "^(get_commit_tree|get_commit_tree_in_graph|load_tree_for_commit)$";
 expression c;
 @@
+  f(...) {...
 - c->maybe_tree
 + get_commit_tree(c)
+  ...}
 
 @@
 expression c;
@@ -22,9 +26,3 @@ expression s;
 @@
 - get_commit_tree(c) = s
 + c->maybe_tree = s
-
-@@
-expression c;
-@@
-- return get_commit_tree(c);
-+ return c->maybe_tree;