Read translation history from repo

[richardolsson]

The Git repo contains the history of all translations (and messages), which can be read by stepping through the Git history and reading the state at each point.

The history would need to be kept per message, even if it's assembled by reading entire files at each point in the Git history.

Knowing the history would allow us, for example, to:

• Indicate to a user when the default text of a message has been changed after translations, and that the translations may need updating
• Let the user see and revert to old translations, e.g. after a mistake
• More intelligently merge an updated Git repo with the cached store, e.g. to replace stored translations with more recent updates from git