t3030: fix accidental success in symlink rename

t3030: fix accidental success in symlink rename

In this test, we have merge two branches. On one branch, we
renamed "a" to "e". On the other, we renamed "a" to "e" and
then added a symlink pointing at "a" pointing to "e".

The results for the test indicate that the merge should
succeed, but also that "a" should no longer exist. Since
both sides renamed "a" to the same destination, we will end
up comparing those destinations for content.

But what about what's left? One side (the rename only),
replaced "a" with nothing. The other side replaced it with a
symlink. The common base must also be nothing, because any
"a" before this was meaningless (it was totally unrelated
content that ended up getting renamed).

The only sensible resolution is to keep the symlink. The
rename-only side didn't touch the content versus the common
base, and the other side added content. The 3-way merge
dictates that we take the side with a change.

And this gives the overall merge an intuitive result.  One
side made one change (a rename), and the other side made two
changes: an identical rename, and an addition (that just
happened to be at the same spot). The end result should
contain both changes.

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