command: unify --job option & default values

Extend the Command class to support adding the --jobs option to the
parser if the command declares it supports running in parallel.  Also
pull the default value used for the number of local jobs into the
command module so local commands can share it.

Change-Id: I22b0f8d2cf69875013cec657b8e6c4385549ccac
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297024
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>

command.py

@@ -23,6 +23,11 @@ from error import NoSuchProjectError
 from error import InvalidProjectGroupsError
 
 
+# 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)
+
+
 class Command(object):
   """Base class for any command line action in repo.
   """
@@ -32,6 +37,10 @@ class Command(object):
   manifest = None
   _optparse = None
 
+  # Whether this command supports running in parallel.  If greater than 0,
+  # it is the number of parallel jobs to default to.
+  PARALLEL_JOBS = None
+
   def WantPager(self, _opt):
     return False
 
@@ -72,6 +81,11 @@ class Command(object):
   def _Options(self, p):
     """Initialize the option parser.
     """
+    if self.PARALLEL_JOBS is not None:
+      p.add_option(
+          '-j', '--jobs',
+          type=int, default=self.PARALLEL_JOBS,
+          help='number of jobs to run in parallel (default: %s)' % self.PARALLEL_JOBS)
 
   def _RegisteredEnvironmentOptions(self):
     """Get options that can be set from environment variables.