[GUILT PATCH 2/5] guilt-guard: Assign guards to patches in series

[GUILT PATCH 2/5] guilt-guard: Assign guards to patches in series

guilt-guard will assign guards to a patch.  They work so that:

    * Patches with no guards are always pushed.

    * Patches with positive guards (i.e. +foo) are pushed *only if* the
      guard is selected.

    * Patches with negative guards (i.e. -foo) are pushed *unless* the
      guard is selected.

This also introduces a number of guard-related utility functions into
guilt.  get_guarded_series is used to get the list of patches that are
applicable given the current guard status.  It replaces get_series in
a later patch.

[jeffpc: small cleanups & bugfixes]
Signed-off-by: Eric Lesh <eclesh@ucla.edu>
Signed-off-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>