Make git_index_read_tree preserve stat cache

Instead of just blowing away the stat cache data when loading a
new tree into the index, this checks if each loaded item has a
corresponding existing item with the same OID and if so, copies
the stat data from the old item to the new one so it will not be
blown away.
Merge pull request #1630 from arrbee/index-read-tree-preserve-stat-cache

Make git_index_read_tree preserve stat cache where possible