windows: check target type before actual unlinking to follow POSIX semantics Creation and writing into target file via vfs (a.k.a opener) is done after "unlink()" target file, if it exists. For example, it is assumed that the revision X consists of file 'A', and the revision Y consists of file 'A/B'. Merging revision X into Y tries to "unlink()" on directory 'A' of 'A/B', before creation of file 'A'. On POSIX environment, directories should be removed by "rmdir(2)", and "unlink(2)" on directories fails. "unlink()" of Mercurial (and Python) uses "unlink(2)" directly, so unlinking in the merge case above would fail. In the other hand, on Windows environment, "unlink()" of Mercurial tries to rename before actual unlinking, to follow POSIX semantics: already opened file can be unlinked safely. This causes unexpected success in unlinking in the merge case above, even though directory 'A' is renamed to another. This confuses users. This patch checks whether target is directory or not before renaming, and raises IOError(errno.EPERM) if so, to follow POSIX semantics.