diff- and log- family: handle "git cmd -h" early

diff- and log- family: handle "git cmd -h" early

"git $builtin -h" bypasses the usual repository setup and calls the
cmd_$builtin() function, expecting it to show the help text.

Unfortunately the commands in the log- and the diff- family want to
call into the revisions machinery, which by definition needs to have
a repository already discovered.  Strictly speaking, they may not
need a repository only for parsing "-h", but it is a good discipline
to future-proof codepath to ensure that setup_revisions() is called
after we know that a repository is there.

Handle the "git $builtin -h" special case very early in these
commands to work around potential issues.

Signed-off-by: Junio C Hamano <gitster@pobox.com>