Revert 4678f4b and provide another solution for bug 467631

Revert 4678f4b and provide another solution for bug 467631

Making gitlinks and folders match in a tree walk was the wrong
approach to fix bug 467631. The problem is that in such a conflict
the tree walk may then not descend into the folder.

Revert the changes to Paths.java and PathsTest.java from commit
4678f4b. Instead test for the problem case from bug 467631 explicitly
in IndexDiff. Add Daniel's test case from bug 545162, and add yet
another test case for DiffEntry.scan() that covers the problem
originally reported in bug 545162.

Bug: 545162
Change-Id: Ie2214c5d5ee32ac6596b621f0f1c7b86d38fa9b7
Also-by: Daniel Veihelmann <daniel.veihelmann@gmail.com>
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>