"git checkout" branch switching safety broken in 'next'

[Junio C Hamano <gitster@pobox.com>]

Linus, please be careful when switching branches if you are using your
unpack_trees() patch (in 'next') for production.  There is a breakage that
makes switching branches with "git checkout" lose your work-in-progress.

Traditionally, when you have a change in the work tree, and switch to
another branch that has different contents in the modified path, we
errored out by saying:

    error: Entry 'foo' not uptodate. Cannot merge.

The updated unpack_trees() notices the condition, issues this message
correctly and tries to propagate the error up, but loses the error at a
few places in the call chain.  The end result is that the caller does not
notice the two-way merge failure, and after that all h*ll breaks loose.

I think this should fix it.

-- >8 --
traverse_trees_recursive(): propagate merge errors up

There were few places where merge errors detected deeper in the call chain
were ignored and not propagated up the callchain to the caller.

Most notably, this caused "git checkout" to switch branches to internally
notice that a path modified in a work tree are different between the HEAD
version and the commit being switched to, but ignore that and switch
branches nevertheless, resulting in an incorrect two-way merge and loss of
the change in the work tree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 unpack-trees.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/unpack-trees.c b/unpack-trees.c
index da68557..5a0f038 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -144,8 +144,7 @@ int traverse_trees_recursive(int n, unsigned long dirmask, unsigned long df_conf
 			sha1 = names[i].sha1;
 		fill_tree_descriptor(t+i, sha1);
 	}
-	traverse_trees(n, t, &newinfo);
-	return 0;
+	return traverse_trees(n, t, &newinfo);
 }
 
 /*
@@ -306,7 +305,9 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 			if (src[0])
 				conflicts |= 1;
 		}
-		traverse_trees_recursive(n, dirmask, conflicts, names, info);
+		if (traverse_trees_recursive(n, dirmask, conflicts,
+					     names, info) < 0)
+			return -1;
 		return mask;
 	}