Add failing test case
Reflog: correct "new" id for reattaching HEAD
Check for errors when dereferencing symbolic refs
Add `git_reference_is_note`.
Merge pull request #2098 from arthurschreiber/arthur/add-git_reference_is_note

Add `git_reference_is_note`.
Merge pull request #2095 from libgit2/update-head-reflog

Correct "new" id for reattached-HEAD reflog entry
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