revision: introduce rev_walk_mode to clarify get_revision_1()

revision: introduce rev_walk_mode to clarify get_revision_1()

get_revision_1() dispatches to different walk strategies based on a
combination of rev_info flags: reflog_info, topo_walk_info, and
limited.  These conditions are checked in multiple places within
the function -- once to select the next commit, and again to decide
how to expand parents -- and the two chains must stay in sync.

Extract the mode selection into a rev_walk_mode enum and a small
get_walk_mode() helper, resolved once at the top of get_revision_1().
Both dispatch sites now switch on the same mode variable, making it
obvious that they agree and easier to verify that all modes are
handled.

No functional change.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>