git-fast-import.txt: clarify that multiple merge commits are allowed

The grammar for commits used a '?' rather than a '*' on the `merge` directive line, despite the fact that the code allows multiple `merge` directives in order to support n-way merges. In fact, elsewhere in git-fast-import.txt there is an explicit declaration that "an unlimited number of `merge` commands per commit are permitted by fast-import". Fix the grammar to match the intent and implementation. Reported-by: Joachim Klein <joachim.klein@automata.tools> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>