init: refactor the template directory discovery into its own function

We will need to call this function from `hook.c` to be able to prevent
hooks from running that were written as part of a `clone` but did not
originate from the template directory.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

cache.h

@@ -656,6 +656,7 @@ int path_inside_repo(const char *prefix, const char *path);
 #define INIT_DB_QUIET 0x0001
 #define INIT_DB_EXIST_OK 0x0002
 
+const char *get_template_dir(const char *option_template);
 int init_db(const char *git_dir, const char *real_git_dir,
 	    const char *template_dir, int hash_algo,
 	    const char *initial_branch, unsigned int flags);