Set GIT_DIR when calling out to Git from the Makefile

This is necessary for out-of-source builds to work.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Missing translation source file

I'm still not 100% sure that this is the way to do it but ran out of time
- and right now the build complains that this file is missing.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>