Documentation: exclude irrelevant options from "git pull"

10eb64f5 (git pull manpage: don't include -n from fetch-options.txt, 2008-01-25) introduced a way to exclude some parts of included source when building git-pull documentation, and later 409b8d82 (Documentation/git-pull: put verbosity options before merge/fetch ones, 2010-02-24) attempted to use the mechanism to exclude some parts of merge-options.txt when used from git-pull.txt. However, the latter did not have an intended effect, because the macro "git-pull" used to decide if the source is included in git-pull documentation were defined a bit too late. Define the macro before it is used to fix this. Signed-off-by: Junio C Hamano <gitster@pobox.com>