t1410: fix breakage on case-insensitive filesystems

Two tests recently added to t1410 create branches "a" and
"a/b" to test d/f conflicts on reflogs. Earlier, unrelated
tests in that script create the path "A/B" in the working
tree.  There's no conflict on a case-sensitive filesystem,
but on a case-insensitive one, "git log" will complain that
"a/b" is both a revision and a working tree path.

We could fix this by using a "--" to disambiguate, but we
are probably better off using names that are less confusing
to make it more clear that they are unrelated to the working
tree files.  This patch turns "a/b" into "one/two".

Reported-by: Michael Blume <blume.mike@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>