When creating a filebuf, detect a directory that exists in our
target file location. This prevents a failure later, when we try
to move the lock file to the destination.
We create a lockfile to update files under GIT_DIR. Sometimes these
files are actually located elsewhere and a symlink takes their place. In
that case we should lock and update the file at its final location
rather than overwrite the symlink.
When we have an error renaming the lockfile, we need to make sure
that we remove it upon cleanup. For this, we need to keep track of
whether we opened the file and whether the rename succeeded.
If we did create the lockfile but the rename did not succeed, we
remove the lockfile. This won't protect against all errors, but
the most common ones (target file is open) does get handled.
