Bug #1022

- Fixed bug #1022, where dependencies were note released when a blocking task
  was completed (thanks to Arkady Grudzinsky).
- The Task object now caches ::is_blocked and ::is_blocking Booleans that are
  determined on pending.data load.
- Simplified and sped up color rule processing using cached values, reducing
  the number of map lookups, and removed loop invariants when the rules are
  not defined.
- Simplified urgency calculations given the cached values for blocked/blocking.
- On load, pending.data is scanned for accurate blocked/blocking status
  determination.
- Obsoleted and removed complex single-task dependency calculations.
- Sped up 'nag' processing by using cached values..
- Modified the 'show' command to consider color.blocking to be valid.
- Added default config value for color.blocking, and included it in the
  precedence list ahead of blocked, as it is more important.
- Updated taskrc.5 man page to include the new color.blocking rule, and its
  place in the rule precedence.

doc/man/taskrc.5.in

@@ -741,6 +741,9 @@ Task is started, therefore active.
 .B color.scheduled
 Task is scheduled, therefore ready for work.
 .br
+.B color.blocking
+Task is blocking another in a dependency.
+.br
 .B color.blocked
 Task is blocked by a dependency.
 .br
@@ -935,7 +938,7 @@ Colors the output of the merge command.
 .RE
 
 .TP
-.B rule.precedence.color=due.today,active,blocked,overdue,due,scheduled,keyword.,project.,tag.,recurring,pri,tagged,completed,deleted
+.B rule.precedence.color=due.today,active,blocking,blocked,overdue,due,scheduled,keyword.,project.,tag.,recurring,pri,tagged,completed,deleted
 .RS
 This setting specifies the precedence of the color rules, from highest to
 lowest.  Note that the prefix 'color.' is omitted (for brevity), and that any