Retain file length and mod time when doing a mixed reset

Previously the index was cleared and updated with a new tree.

Now the commit being reset to and the index are iterated over
in a tree walk and the current index mod time and file length
are copied over to the new dir cache entry being written if
the object ids are the same.

Change-Id: Iaf9e624efb0bf90f9e05fcb0587dde4dec50000c