revisions: split out handle_revision_pseudo_opt function

revisions: split out handle_revision_pseudo_opt function

As v1.6.0-rc2~42 (Allow "non-option" revision options in
parse_option-enabled commands, 2008-07-31) explains, options handled
by setup_revisions fall into two categories:

 1. global options like --topo-order handled by parse_revision_opt,
    which can take detached arguments and can be parsed in advance;

 2. pseudo-options that must be parsed in order with their revision
    counterparts, like --not and --all.

The global options are taken care of by handle_revision_opt; the
pseudo-options are currently in a deeply indented portion of
setup_revisions.  Give them their own function for easier reading.

The only goal is to make setup_revisions easier to read straight
through.  No functional change intended.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>