Fix IndexDiffs for git links

Fix IndexDiffs for git links

After cloning a repo with a submodule, non-recursively, JGit would
encounter in its TreeWalk in IndexDiff:

* first, a missing gitlink (in index & HEAD, not in working tree)
* second, the untracked folder (not in index and head, in working tree)

As a result, it would report the submodule as missing. Canonical git
reports a clean workspace.

The root cause of this is that the path of a gitlink "x" did
not compare equal to the path of a tree "x" in JGit.

Correct Paths.compare() to account for that. If two paths are otherwise
equal, then let gitlinks match both trees and files. Matching trees
solves the bug. Matching files is necessary to handle the case where
the gitlink directory was replaced by a file; see the new test case
IndexDiffSubmoduleTest.testSubmoduleReplacedByFile(). Comparisons of
unequal paths are left untouched, so the sort order is unchanged.

After the fix, another bug(?) in WorkingTreeIterator became apparent:
with core.dirNoGitLinks = true, it was no longer possible to overwrite
a gitlink in the index. This is now fixed in WorkingTreeIterator.

Add new test cases for the bug itself and for some related cases
(submodule directory deleted or replaced by a file) in
IndexDiffSubmoduleTest. Add a test for missing files in IndexDiffTest,
and adapt the PathsTest to test matching gitlinks.

Bug: 467631
Change-Id: I0549d10d46b1858e5eec3cc15095aa9f1d5f5280
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>