checkout: rollback lock on early returns in merge_working_tree

checkout: rollback lock on early returns in merge_working_tree

merge_working_tree() acquires the index lock via
repo_hold_locked_index() but several early return paths exit
without calling rollback_lock_file(), leaving the lock held.
While this is currently harmless because the process exits soon
after, it becomes a problem if the function is ever called more
than once in the same process.

Add rollback_lock_file() calls to all early return paths.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>