Demonstrate bugs when a directory is replaced with a symlink

Demonstrate bugs when a directory is replaced with a symlink

This test creates two directories, a/b and a/b-2, then replaces a/b with
a symlink to a/b-2, then merges that change into the 'baseline' commit,
which contains an unrelated change.

There are two bugs:
1. 'git checkout' incorrectly deletes work tree file a/b-2/d.
2. 'git merge' incorrectly deletes work tree file a/b-2/d.

The test goes on to create another branch in which a/b-2 is replaced
with a symlink to a/b (i.e., the reverse of what was done the first
time), and merge it into the 'baseline' commit.

There is a different bug:
3. The merge should be clean, but git reports a conflict.

Signed-off-by: James Pickens <james.e.pickens@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>