t1092: document bad 'git checkout' behavior

Add new branches to the test repo that demonstrate directory/file
conflicts in different ways. Since the directory 'folder1/' has
adjacent files 'folder1-', 'folder1.txt', and 'folder10' it causes
searches for 'folder1/' to land in a different place in the index than a
search for 'folder1'. This causes a change in behavior when working with
the df-conflict-1 and df-conflict-2 branches, whose only difference is
that the first uses 'folder1' as the conflict and the other uses
'folder2' which does not have these adjacent files.

We can extend two tests that compare the behavior across different 'git
checkout' commands, and we see already that the behavior will be
different in some cases and not in others. The difference between the
two test loops is that one uses 'git reset --hard' between iterations.

Further, we isolate the behavior of creating a staged change within a
directory and then checking out a branch where that directory is
replaced with a file. A full checkout behaves differently across these
two cases, while a sparse-checkout cone behaves consistently. In both
cases, the behavior is wrong. In one case, the staged change is dropped
entirely. The other case the staged change is kept, replacing the file
at that location, but none of the other files in the directory are kept.

Likely, the correct behavior in this case is to reject the checkout and
report the conflict, leaving HEAD in its previous location. None of the
cases behave this way currently. Use comments to demonstrate that the
tested behavior is only a documentation of the current, incorrect
behavior to ensure we do not _accidentally_ change it. Instead, we would
prefer to change it on purpose with a future change.

At this point, the sparse-index does not handle these 'git checkout'
commands correctly. Or rather, it _does_ reject the 'git checkout' when
we have the staged change, but for the wrong reason. It also rejects the
'git checkout' commands when there is no staged change and we want to
replace a directory with a file. A fix for that unstaged case will
follow in the next change, but that will make the sparse-index agree
with the full checkout case in these documented incorrect behaviors.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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