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>

subcmds/sync.py

@@ -177,12 +177,14 @@ If the remote SSH daemon is Gerrit Code Review, version 2.0.10 or
 later is required to fix a server side protocol bug.
 
 """
+  PARALLEL_JOBS = 1
 
   def _Options(self, p, show_smart=True):
     try:
-      self.jobs = self.manifest.default.sync_j
+      self.PARALLEL_JOBS = self.manifest.default.sync_j
     except ManifestParseError:
-      self.jobs = 1
+      pass
+    super()._Options(p)
 
     p.add_option('-f', '--force-broken',
                  dest='force_broken', action='store_true',
@@ -222,9 +224,6 @@ later is required to fix a server side protocol bug.
     p.add_option('-q', '--quiet',
                  dest='output_mode', action='store_false',
                  help='only show errors')
-    p.add_option('-j', '--jobs',
-                 dest='jobs', action='store', type='int',
-                 help="projects to fetch simultaneously (default %d)" % self.jobs)
     p.add_option('-m', '--manifest-name',
                  dest='manifest_name',
                  help='temporary manifest to use for this sync', metavar='NAME.xml')