Include checkpoint command in the BNF.

This command isn't encouraged (as its slow) but it does exist and
is accepted, so it still should be covered in the BNF.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -7,6 +7,7 @@ Format of STDIN stream:
         | new_commit
         | new_tag
         | reset_branch
+        | checkpoint
         ;
 
   new_blob ::= 'blob' lf