avoid nanosleep(2)

avoid nanosleep(2)

On Solaris nanosleep(2) is not available in libc; you need to
link with -lrt to get it.

The purpose of the loop is to wait until the next filesystem
timestamp granularity, and the code uses subsecond sleep in the
hope that it can shorten the delay to 0.5 seconds on average
instead of a full second.  It is probably not worth depending on
an extra library for this.

We might want to yank out the whole "racy-git avoidance is
costly later at runtime, so let's delay writing the index out"
codepath later, but that is a separate issue and needs some
testing on large trees to figure it out.  After playing with the
kernel tree, I have a feeling that the whole thing may not be
worth it.

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