Fix processing of translated messages in `git-artifacts`

[dscho]

I noticed that recent git-artifacts builds in git-sdk-64 started outputting lines like this:

/usr/bin/makepkg: line 1023: /usr/bin/gettext: No such file or directory

The reason is most likely that I recently changed Git for Windows not to include gettext, and the makepkg-git sparse-checkout definition needing to account for that.

A probable consequence of this issue is that the translated messages (which are not shipped in Git for Windows' installer, to save on bandwidth) are either broken or not even included in the mingw-w64-git package anymore.

[dscho]

Turns out that git-for-windows/build-extra#560 only addressed the i686 build, but not the x86_64 one. I fixed this via git-for-windows/git-sdk-64@a6f190c.