object_id.cocci: match only expressions of type 'struct object_id'

object_id.cocci: match only expressions of type 'struct object_id'

Most of our semantic patches in 'contrib/coccinelle/object_id.cocci'
turn calls of SHA1-specific functions into calls of their
corresponding object_id counterparts, e.g. sha1_to_hex() to
oid_to_hex().  These semantic patches look something like this:

  @@
  expression E1;
  @@
  - sha1_to_hex(E1.hash)
  + oid_to_hex(&E1)

and match the access to the 'hash' field in any data type, not only in
'struct object_id', and, consquently, can produce wrong
transformations.

Case in point is the recent hash function transition patch "rerere:
convert to use the_hash_algo" [1], which, among other things, renamed
'struct rerere_dir's 'sha1' field to 'hash', and then 'make
coccicheck' started to suggest the following wrong transformations for
'rerere.c' [2]:

  -    return sha1_to_hex(id->collection->hash);
  +    return oid_to_hex(id->collection);

and

  -    DIR *dir = opendir(git_path("rr-cache/%s", sha1_to_hex(rr_dir->hash)));
  +    DIR *dir = opendir(git_path("rr-cache/%s", oid_to_hex(rr_dir)));

Avoid such wrong transformations by tightening semantic patches in
'object_id.cocci' to match only type of or pointers to 'struct
object_id'.

[1] https://public-inbox.org/git/20181008215701.779099-15-sandals@crustytoothpaste.net/
[2] https://travis-ci.org/git/git/jobs/440463476#L580

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>