redo-log: capture and linearize the output of redo builds.

redo now saves the stderr from every .do script, for every target, into
a file in the .redo directory.  That means you can look up the logs
from the most recent build of any target using the new redo-log
command, for example:

	redo-log -r all

The default is to show logs non-recursively, that is, it'll show when a
target does redo-ifchange on another target, but it won't recurse into
the logs for the latter target.  With -r (recursive), it does.  With -u
(unchanged), it does even if redo-ifchange discovered that the target
was already up-to-date; in that case, it prints the logs of the *most
recent* time the target was generated.

With --no-details, redo-log will show only the 'redo' lines, not the
other log messages.  For very noisy build systems (like recursing into
a 'make' instance) this can be helpful to get an overview of what
happened, without all the cruft.

You can use the -f (follow) option like tail -f, to follow a build
that's currently in progress until it finishes.  redo itself spins up a
copy of redo-log -r -f while it runs, so you can see what's going on.

Still broken in this version:

- No man page or new tests yet.

- ANSI colors don't yet work (unless you use --raw-logs, which gives
  the old-style behaviour).

- You can't redirect the output of a sub-redo to a file or a
  pipe right now, because redo-log is eating it.

- The regex for matching 'redo' lines in the log is very gross.
  Instead, we should put the raw log files in a more machine-parseable
  format, and redo-log should turn that into human-readable format.

- redo-log tries to "linearize" the logs, which makes them
  comprehensible even for a large parallel build.  It recursively shows
  log messages for each target in depth-first tree order (by tracing
  into a new target every time it sees a 'redo' line).  This works
  really well, but in some specific cases, the "topmost" redo instance
  can get stuck waiting for a jwack token, which makes it look like the
  whole build has stalled, when really redo-log is just waiting a long
  time for a particular subprocess to be able to continue.  We'll need to
  add a specific workaround for that.

vars.py

@@ -16,6 +16,7 @@ DEBUG_PIDS = os.environ.get('REDO_DEBUG_PIDS', '') and 1 or 0
 VERBOSE = os.environ.get('REDO_VERBOSE', '') and 1 or 0
 XTRACE = os.environ.get('REDO_XTRACE', '') and 1 or 0
 KEEP_GOING = os.environ.get('REDO_KEEP_GOING', '') and 1 or 0
+RAW_LOGS = os.environ.get('REDO_RAW_LOGS', '') and 1 or 0
 SHUFFLE = os.environ.get('REDO_SHUFFLE', '') and 1 or 0
 STARTDIR = os.environ.get('REDO_STARTDIR', '')
 RUNID = atoi(os.environ.get('REDO_RUNID')) or None