init, clone: support --separate-git-dir for .git file

--separate-git-dir tells git to create git dir at the specified
location, instead of where it is supposed to be. A .git file that
points to that location will be put in place so that it appears normal
to repo discovery process.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -436,6 +436,7 @@ extern void verify_non_filename(const char *prefix, const char *name);
 
 #define INIT_DB_QUIET 0x0001
 
+extern int set_git_dir_init(const char *git_dir, const char *real_git_dir, int);
 extern int init_db(const char *template_dir, unsigned int flags);
 
 #define alloc_nr(x) (((x)+16)*3/2)