rebase (autostash): store the full OID in <state-dir>/autostash

It was reported by Gábor Szeder and analyzed by Alban Gruin that the
built-in rebase stores only abbreviated stash hashes in the `autostash`
file.

This is problematic e.g. in t5520-pull.sh, where the abbreviated hash is
so short that it sometimes consists only of digits, which are
subsequently mistaken ("DWIMmed") for numbers by `git stash apply`.

Let's align the behavior of the built-in rebase with the scripted rebase
and store the full stash hash instead. That makes it a lot less likely
that it consists only of digits.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1374,7 +1374,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			if (safe_create_leading_directories_const(autostash))
 				die(_("Could not create directory for '%s'"),
 				    options.state_dir);
-			write_file(autostash, "%s", buf.buf);
+			write_file(autostash, "%s", oid_to_hex(&oid));
 			printf(_("Created autostash: %s\n"), buf.buf);
 			if (reset_head(&head->object.oid, "reset --hard",
 				       NULL, 0, NULL, NULL) < 0)