t6022: Add tests for reversing order of merges when D/F conflicts present

t6022: Add tests for reversing order of merges when D/F conflicts present

When merging two branches with some path involved in a D/F conflict, the
choice of which branch to merge into the other matters for (at least) two
reasons: (1) whether the working copy has a directory full of files that
is in the way of a file, or a file exists that is in the way of a
directory of files, (2) when the directory full of files does not disappear
due to the merge, what files at the same paths should be renamed to
(e.g. filename~HEAD vs. filename~otherbranch).

Add some tests that reverse the merge order of two other tests, and which
verify the contents are as expected (namely, that the results are identical
other than modified-for-uniqueness filenames involving branch names).

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>