Fix crashes on rare combination of file names

Fix crashes on rare combination of file names

The NameConflictTreeWalk class is used in merge for iterating over
entries in commits. The class uses a separate iterator for each
commit's tree. In rare cases it can incorrectly report the same entry
twice. As a result, duplicated entries are added to the merge result
and later jgit throws an exception when it tries to process merge
result.

The problem appears only when there is a directory-file conflict for
the last item in trees. Example from the bug:
Commit 1:
* subtree - file
* subtree-0 - file
Commit 2:
* subtree - directory
* subtree-0 - file
Here the names are ordered like this:
"subtree" file <"subtree-0" file < "subtree" directory.

The NameConflictTreeWalk handles similar cases correctly if there are
other files after subtree... in commits - this is processed in the
AbstractTreeIterator.min function. Existing code has a special
optimization for the case, when all trees are pointed to the same
entry name - it skips additional checks. However, this optimization
incorrectly skips checks if one of trees reached the end.

The fix processes a situation when some trees reached the end, while
others are still point to an entry.

bug: 535919
Change-Id: I62fde3dd89779fac282479c093400448b4ac5c86