build-extra: rename the `git-extra/` directory

[dscho]

As I found out the hard way in git-for-windows/build-extra#456, we cannot yet call /deploy mingw-w64-git-extra because that expects the directory name, and that name is still git-extra.

Addressing this issue won't be quite as easy as simple renaming the directory: there are at least some references in git-sdk.sh, and potentially also in please.sh that have to be vetted.