lockfile: allow reopening a closed but still locked file

In some code paths (e.g. giving "add -i" to prepare the contents to
be committed interactively inside "commit -p") where a caller takes
a lock, writes the new content, give chance for others to use it
while still holding the lock, and then releases the lock when all is
done.  As an extension, allow the caller to re-update an already
closed file while still holding the lock (i.e. not yet committed) by
re-opening the file, to be followed by updating the contents and
then by the usual close_lock_file() or commit_lock_file().

This is necessary if we want to add code to rebuild the cache-tree
and write the resulting index out after "add -i" returns the control
to "commit -p", for example.

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

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