fast-import: stop creating leading directories for import-marks

fast-import: stop creating leading directories for import-marks

When asked to import marks from "subdir/file.marks", we create the
leading directory "subdir" if it doesn't exist. This makes no sense for
importing marks, where we only ever open the path for reading.

Most of the time this would be a noop, since if the marks file exists,
then the leading directories exist, too. But if it doesn't (e.g.,
because --import-marks-if-exists was used), then we'd create the useless
directory.

This dates back to 580d5f83e7 (fast-import: always create marks_file
directories, 2010-03-29). Even then it was useless, so it seems to have
been added in error alongside the --export-marks case (which _is_
helpful).

Signed-off-by: Jeff King <peff@peff.net>