close_lock_file(): new function in the lockfile API

close_lock_file(): new function in the lockfile API

The lockfile API is a handy way to obtain a file that is cleaned
up if you die().  But sometimes you would need this sequence to
work:

 1. hold_lock_file_for_update() to get a file descriptor for
    writing;

 2. write the contents out, without being able to decide if the
    results should be committed or rolled back;

 3. do something else that makes the decision --- and this
    "something else" needs the lockfile not to have an open file
    descriptor for writing (e.g. Windows do not want a open file
    to be renamed);

 4. call commit_lock_file() or rollback_lock_file() as
    appropriately.

This adds close_lock_file() you can call between step 2 and 3 in
the above sequence.

Signed-off-by: Junio C Hamano <gitster@pobox.com>