cache: use const char * for get_object_directory()

The get_object_directory() method returns the exact string stored at
the_repository->objects->odb->path. The return type of "char *" implies
that the caller must keep track of the buffer and free() it when
complete. This causes significant problems later when the ODB is
accessed.

Use "const char *" as the return type to avoid this confusion. There are
no current callers that care about the non-const definition.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>

cache.h

@@ -566,7 +566,7 @@ extern char *git_work_tree_cfg;
 int is_inside_work_tree(void);
 const char *get_git_dir(void);
 const char *get_git_common_dir(void);
-char *get_object_directory(void);
+const char *get_object_directory(void);
 char *get_index_file(void);
 char *get_graft_file(struct repository *r);
 void set_git_dir(const char *path, int make_realpath);

environment.c

@@ -273,7 +273,7 @@ const char *get_git_work_tree(void)
 	return the_repository->worktree;
 }
 
-char *get_object_directory(void)
+const char *get_object_directory(void)
 {
 	if (!the_repository->objects->odb)
 		BUG("git environment hasn't been setup");