commands: document the "common" class attribute

Switch it to uppercase to make it clear it's a constant, and add
documentation so its usage is clear.

Change-Id: I6d281a66a90b5908b3131585c9945e88cfe815ea
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/309322
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

@@ -66,7 +66,7 @@ _ONE_DAY_S = 24 * 60 * 60
 
 class Sync(Command, MirrorSafeCommand):
   jobs = 1
-  common = True
+  COMMON = True
   helpSummary = "Update working tree to the latest revision"
   helpUsage = """
 %prog [<project>...]