I'd thought it was somewhat agnostic about which VCS it used. I doubt I'll switch now - a quick google shows it is possible to convert, but it's more hassle than I can be bothered with.
It's working fine with bzr, and it's Friday. So to change it breaks the two golden rules:
* It Ain't Broken, So Don't Fix It
* Don't Fiddle[1] With It On Friday
Thanks for pointing that out though - it's good to know I may need to do it in the future, if bzr support ever wanes for the etckeeper project...
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[1] Alternative verbs have seen use at this position.