subcmds: stop instantiating at import time

The current subcmds design has singletons in all_commands.  This isn't
exactly unusual, but the fact that our main & help subcommand will then
attach members to the classes before invoking them is.  This makes it
hard to keep track of what members a command has access to, and the two
code paths (main & help) attach different members depending on what APIs
they then invoke.

Lets pull this back a step by storing classes in all_commands and leave
the instantiation step to when they're used.  This doesn't fully clean
up the confusion, but gets us closer.

Change-Id: I6a768ff97fe541e6f3228358dba04ed66c4b070a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259154
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

main.py

@@ -204,7 +204,7 @@ class _Repo(object):
     SetDefaultColoring(gopts.color)
 
     try:
-      cmd = self.commands[name]
+      cmd = self.commands[name]()
     except KeyError:
       print("repo: '%s' is not a repo command.  See 'repo help'." % name,
             file=sys.stderr)