fast-import: allow for multiple --import-marks= arguments

fast-import: allow for multiple --import-marks= arguments

The --import-marks= option may be specified multiple times on the
commandline and should result in all marks being read in. Only one
import-marks feature may be specified in the stream, which is
overriden by any --import-marks= commandline options.

If one wishes to specify import-marks files in addition to the one
specified in the stream, it is easy to repeat the stream option as a
--import-marks= commandline option.

Also verify this behavior with tests.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>