sha1_name.c: fix parsing of ":/token" syntax

sha1_name.c: fix parsing of ":/token" syntax

The parser tried to clean up the object flags it used while finding
commits with matching string, but was not doing a very good job at it.

This caused "checkout -b new ':/token'", which internally tries to parse
':/token' twice as an object name, to fail when the commit in question
was reachable from only one ref.

The mask bits given to pop_most_recent_commit(&list, MASK) means "I have
already been on the list to be processed, so please do not place me again
even if I am found to be a parent of some other commit on the list."  So
mark them when we add them to the list at the beginning.

Signed-off-by: Junio C Hamano <gitster@pobox.com>