Set author and committer at git init

Fixes #1747
- we use init rather than clone to decouple init from update
- we need HEAD defined to be able to diff, etc, for that an init commit is needed
- anyway after that we'll reset to the master branch

this defines a bunch of env variables so that git doesn't complain.
Merge pull request #1756 from OCamlPro/fix-git-init

Set author and committer at git init