[PATCH 0 of 7 STABLE] merge: fix problems of collision between files, symbolic-links and directories

[FUJIWARA Katsunori]

This patch series fixes problems of collision between files,
symbolic-links and directories at merging/updating.

Problem fixed by patch #1 may never occur, because patch #2 (for
issue29) avoids actual unlinking on directories in updating/merging.
But IMHO it should be fixed, to prevent difference from POSIX
semantics from causing problems in the future.