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>

Documentation/git-fast-import.txt

@@ -390,7 +390,7 @@ change to the project.
 	'committer' (SP <name>)? SP LT <email> GT SP <when> LF
 	data
 	('from' SP <commit-ish> LF)?
-	('merge' SP <commit-ish> LF)?
+	('merge' SP <commit-ish> LF)*
 	(filemodify | filedelete | filecopy | filerename | filedeleteall | notemodify)*
 	LF?
 ....