+ support i18n submodule updates

Because i18n and core/compat are handled by separate processes, we need
to fiddle a bit to get things working. This is the rough method:

After each i18n commit:
  - start tailing log_i18n
  - checkpoint
  - using log file, make sure checkpoint is finished
  - get latest revision from git log and store

Then, at a fixed revision (8833 for core, 8932 for compat), add a
.gitmodules file, mode 100664, contents:

[submodule "i18n"]
    path = i18n
    url = https://gerrit.wikimedia.org/r/p/pywikibot/i18n.git

(path = scripts/i18n for core)

For each revision >= 8833 (core), >= 8932 (compat), hand git fast-import a
modification of scripts/i18n (core) / i18n (compat), as

M 160000 <SHA1> i18n LF

A nicer way would be to implement a commit after a change in i18n, but
at least this works.