Show hg or git commit in build info

[reticulatedpines]

The parent repo stamped builds with hg commit info. I removed this to migrate to git. It would be nice to restore this functionality and extend to git, so the build determines which is managing the source and uses appropriate tools. If neither, don't fail.

See: https://github.com/reticulatedpines/magiclantern_simplified/commit/3163d41cee35fb54b4308e1f32318ff87de53645 https://github.com/reticulatedpines/magiclantern_simplified/commit/4a29a347f29276d4bec0876412cec4bb12b5a28c

[reticulatedpines]

I hacked this in quickly for the 200d release. I haven't closed the ticket because it's not very neat, and again, the build will fail if you have a copy of the source and it's not git managed / you don't have git installed.