Tests merge when changes exist in workdir/index
Tests merging staged files identical to result
Test that emulates a strange filter implementation
Merge wd validation tests against index not HEAD

Validating the workdir should not compare HEAD to working
directory - this is both inefficient (as it ignores the cache)
and incorrect.  If we had legitimately allowed changes in the
index (identical to the merge result) then comparing HEAD to
workdir would reject these changes as different.  Further, this
will identify files that were filtered strangely as modified,
while testing with the cache would prevent this.

Also, it's stupid slow.
Remove unused pointer assignment
Merge pull request #2096 from ethomson/merge

Corrections for merge working directory validation