subcmds: force consistent help text format

We're inconsistent with help text as to whether it uses title case and
whether it ends in a period.  Add a test to enforce a standard, and use
the style that Python optparse & argparse use themselves (e.g. with the
--help option): always lowercase, and never trailing period.

Change-Id: Ic1defae23daeac0ac9116aaf487427f50b34050d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305144
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

@@ -169,10 +169,11 @@ later is required to fix a server side protocol bug.
   PARALLEL_JOBS = 1
 
   def _CommonOptions(self, p):
-    try:
-      self.PARALLEL_JOBS = self.manifest.default.sync_j
-    except ManifestParseError:
-      pass
+    if self.manifest:
+      try:
+        self.PARALLEL_JOBS = self.manifest.default.sync_j
+      except ManifestParseError:
+        pass
     super()._CommonOptions(p)
 
   def _Options(self, p, show_smart=True):