attrcache: remove useless `do_init` indirection

Remove useless indirection from `git_attr_cache__init` to
`git_attr_cache__do_init`. The difference is that the
`git_attr_cache__init` macro first checks if the cache is already
initialized and, if so, not call `git_attr_cache__do_init`. But
actually, `git_attr_cache__do_init` already does the same thing and
returns immediately if the cache is already initialized.

Remove the indirection.

src/attrcache.c

@@ -354,7 +354,7 @@ static void attr_cache__free(git_attr_cache *cache)
 	git__free(cache);
 }
 
-int git_attr_cache__do_init(git_repository *repo)
+int git_attr_cache__init(git_repository *repo)
 {
 	int ret = 0;
 	git_attr_cache *cache = git_repository_attr_cache(repo);

src/attrcache.h

@@ -22,10 +22,7 @@ typedef struct {
 	git_pool  pool;
 } git_attr_cache;
 
-extern int git_attr_cache__do_init(git_repository *repo);
-
-#define git_attr_cache__init(REPO) \
-	(git_repository_attr_cache(REPO) ? 0 : git_attr_cache__do_init(REPO))
+extern int git_attr_cache__init(git_repository *repo);
 
 /* get file - loading and reload as needed */
 extern int git_attr_cache__get(