difftool: only copy back files modified during directory diff

difftool: only copy back files modified during directory diff

When 'difftool --dir-diff' is used to compare working tree files,
it always copies files from the tmp dir back to the working tree
when the diff tool is closed, even if the files were not modified
by the diff tool.

This causes the file timestamp to change. Files should only be
copied from the tmp dir back to the working copy if they were
actually modified.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>