Do not write out new index if nothing has changed.
Do not write out new index if nothing has changed.

The git-update-cache command, especially with --refresh, may not change
anything.  In such a case, writing 1.6MB of the same thing is a waste.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git