build-extra: rename the git-extra/ directory #4236
Description
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.