[PATCH] read-tree: fix too strong index requirement #5ALT

[PATCH] read-tree: fix too strong index requirement #5ALT

This fixes too strong index requirement 3-way merge enforces in
one case: the same file is added in both branches.

In this case, the original code insisted that if the index file
has that path, it must match our branch and be up-to-date.
However in this particular case, it only has to match it, and
can be dirty.  We just need to make sure that we keep the
work-tree copy instead of checking out the merge result.

The resolution of such a path, however, cannot be left to
outside script, because we will not keep the original stage0
entries for unmerged paths when read-tree finishes, and at that
point, the knowledge of "if we resolve it to match the new file
added in both branches, the merge succeeds and the work tree
would not lose information, but we should _not_ update the work
tree from the resulting index file" is lost.  For this reason,
the now code needs to resolve this case (#5ALT) internally.

This affects some existing tests in the test suite, but all in
positive ways.  In t1000 (3-way test), this #5ALT case now gets
one stage0 entry, instead of an identical stage2 and stage3
entry pair, for such a path, and one test that checked for merge
failure (because the test assumed the "stricter-than-necessary"
behaviour) does not have to fail anymore.  In t1005 (emu23
test), two tests that involves a case where the work tree
already had a change introduced in the upstream (aka "merged
head"), the merge succeeds instead of failing.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>