Keep git dir in git build context

[vladaionescu]

Let me know if you can think of any downsides if we do this.

Fixes #813

[alexcb]

I could see this backfiring for users who structure their build in a way that depends on .git, only to discover if they distributed a source-code release as a .tar.gz, the building out of that source code will fail since the .git directory won't exist.

By allowing the .git to be copied into the build we'll ultimately giving the power to the user to decide weather or not the .git contents should be referenced or not.

Perhaps a different solution would be to allow a --strict mode which skips the .git contents from being included. That would make it problematic for users to issue commands like git rev-parse HEAD and would force them to use the earthly env variables instead. That could work for their own build scripts, but what if they wanted to use a vendored tool which required a .git; it seems like a lot to ask users to ditch those tools in favour of (re)writing them to use our env variables (vendor lockin).

Ultimately I think it's fine to include the .git even under the --strict mode.