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

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>