tests/test-merge-subrepos.t

dispatch: make loading extra information from extension extensible

This patch makes loading extra information from extension module at
dispatching extensible. Factoring 'loadcmdtable()' into commands.py is
a part of generalization of loading extra information.

This extensibility assumes registration of new function like below,
for example:

  - revset predicate
  - fileset predicate
  - template keyword
  - template filter
  - template function
  - internal merge tool
  - web command

This patch requires not loader function itself but container module
and the name of it, because listing loader function directly up
implies actual loading module of it, even if it isn't used at runtime
(for example, extensions don't always define revset predicate)

  $ hg init

  $ echo a > a
  $ hg ci -qAm 'add a'

  $ hg init subrepo
  $ echo 'subrepo = http://example.net/libfoo' > .hgsub
  $ hg ci -qAm 'added subrepo'

  $ hg up -qC 0
  $ echo ax > a
  $ hg ci -m 'changed a'
  created new head

  $ hg up -qC 1
  $ cd subrepo
  $ echo b > b
  $ hg add b
  $ cd ..

Should fail, since there are added files to subrepo:

  $ hg merge
  abort: uncommitted changes in subrepository 'subrepo'
  [255]