Most of the callers except the one in refs.c use the function to
update the index file.  Among the index writers, everybody
except write-tree dies if they cannot open it for writing.

This gives the function an extra argument, to tell it to die
when it cannot create a new file as the lockfile.

The only caller that does not have to die is write-tree, because
updating the index for the cache-tree part is optional and not
being able to do so does not affect the correctness.  I think we
do not have to be so careful and make the failure into die() the
same way as other callers, but that would be a different patch.

Signed-off-by: Junio C Hamano <junkio@cox.net>