doc/fast-import: document feature import-marks-if-exists

doc/fast-import: document feature import-marks-if-exists

fast-import command-line option --import-marks-if-exists was introduced
in commit dded4f1 (fast-import: Introduce --import-marks-if-exists, 2011-01-15)

--import-marks option can be set via a "feature" command in a fast-import
stream and --import-marks-if-exists had support for such specification
from the very beginning too due to some shared codebase. Though the
documentation for this feature wasn't written in dded4f1.

Add the documentation for "feature import-marks-if-exists=<file>". Also add
a minimalistic test for it.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>