Documentation: exclude irrelevant options from "git pull"

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>