clone: Supply the right commit hash to post-checkout when -b is used

When we use -b <branch>, we may checkout something else than what the
remote's HEAD references, but we still used remote_head to supply the
new ref value to the post-checkout hook, which is wrong.

So instead of using remote_head to find the value to be passed to the
post-checkout hook, we have to use our_head_points_at, which is always
correctly setup, even if -b is not used.

This also fixes a segfault when "clone -b <branch>" is used with a
remote repo that doesn't have a valid HEAD, as in such a case
remote_head is NULL, but we still tried to access it.
Reported-by: NDevin Cofer <ranguvar@archlinux.us>
Signed-off-by: NBjörn Steinbrink <B.Steinbrink@gmx.de>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-clone.c

@@ -641,7 +641,8 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 			die("unable to write new index file");
 
 		err |= run_hook(NULL, "post-checkout", sha1_to_hex(null_sha1),
-				sha1_to_hex(remote_head->old_sha1), "1", NULL);
+				sha1_to_hex(our_head_points_at->old_sha1), "1",
+				NULL);
 
 		if (!err && option_recursive)
 			err = run_command_v_opt(argv_submodule, RUN_GIT_CMD);

remote-curl.c

@@ -3,6 +3,7 @@
 #include "strbuf.h"
 #include "walker.h"
 #include "http.h"
+#include "exec_cmd.h"
 
 static struct ref *get_refs(struct walker *walker, const char *url)
 {