[git] IDEA-103530 fix view stashed items

1. stash objects are commits with 2 parents (i.e. merge commits).
   One commit is the HEAD you stashed from, and the other is the
   saved state of the index. => we need the first parent.
2. git show --name-status for a merge commit shows only files modified
   in both parents.

http://git.661346.n2.nabble.com/Bug-git-show-behaves-incorrectly-when-called-on-a-stash-object-tc7580152.html#a7580155