status: improve parallel execution stability

The status command runs a bunch of jobs in parallel, and each one
is responsible for writing to stdout directly.  When running many
noisy jobs in parallel, output can get intermingled.  Pass down a
StringIO buffer for writing to so we can return the entire output
as a string so the main job can handle displaying it.  This fixes
interleaved output as well as making the output stable: we always
display results in the same project order now.  By switching from
map to imap, this ends up not really adding any overhead.

Bug: https://crbug.com/gerrit/12231
Change-Id: Ic18b07c8074c046ff36e306eb8d392fb34fb6eca
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297242
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>

command.py

@@ -23,6 +23,15 @@ from error import NoSuchProjectError
 from error import InvalidProjectGroupsError
 
 
+# Number of projects to submit to a single worker process at a time.
+# This number represents a tradeoff between the overhead of IPC and finer
+# grained opportunity for parallelism. This particular value was chosen by
+# iterating through powers of two until the overall performance no longer
+# improved. The performance of this batch size is not a function of the
+# number of cores on the system.
+WORKER_BATCH_SIZE = 32
+
+
 # How many jobs to run in parallel by default?  This assumes the jobs are
 # largely I/O bound and do not hit the network.
 DEFAULT_LOCAL_JOBS = min(os.cpu_count(), 8)